Like wine and art, classic cars are a finite tangible resource often produced in overseas markets that, as investments, can appreciate in value without a correlation to stocks and bonds. Unlike art, you don't have to compete with billionaires to pick up a coveted — and often beautiful — asset in the car market.

The Historic Automobile Group International (HAGI) tracks vintage collectibles with its HAGI Top Index and subindexes. The index was up near-14 percent through the first three quarters of 2015 (the index hit an all-time high in September) and is up more than 500 percent in the past decade. Many of the fastest-appreciating cars are sporty models from Europe.

But which European sports cars make the best investments? We asked HAGI founder Dietrich Hatlapa, specialty-car insurer Hagerty CEO McKeel Hagerty, and RM Sotheby's car specialist David Swig to provide a range of options — from as low as $10,000 to as much as half a million dollars, starting with the most affordable.

(Note: Current value is based on Hagerty.com average pricing data, unless otherwise noted.)

— By Joe D'Allegro, special to CNBC.com
Posted 4 December 2015

It's important to note that "collectible cars are passion investments," said HAGI founder Hatlapa. "There's non-financial value in a classic car — pride of ownership, social recognition, fun, transportation, meeting others at car events — that should ideally outweigh other concerns. If you buy something that makes you happy, you can't really go wrong."

With that out of the way, Hagerty noted that the 1990–2001 SL featured the same hewn-from-granite build quality of earlier Mercedes, but with modernized looks and features.

"These are extremely well made, with great engines," he said. "And an SL that's been well cared for will drive like a modern car, even if it's more than 20 years old."

The SL market bottomed out around 2008–2009, Hagerty said, but it has been rising for the past year. Collectors should check service records before making a purchase for these complex cars that need regular upkeep. Sales prices show the wisdom of this: While poor-condition SLs can be had for less than $8,000, ones in excellent condition go for more than $18,000.

Average price for a 1990 Mercedes-Benz 500SL: $10,950

This elegant sports car is the only Swedish car on our list (sorry Saab), and also the oldest. It was manufactured from 1961 to 1973.

Simple and overbuilt, the P1800 is famously long-lived — one native New Yorker has logged a world-record 3-million-plus miles in his 1966 P1800. Roger Moore fought villains with the help of a white P1800 in six seasons of "The Saint."

"A lot of guys here in San Francisco like them as a stylish older car you can still drive every day," Swig said. "The P1800 is a good entry-level classic."

Average price of a 1962 Volvo P1800: $11,364

Porsche started producing the 911 in 1964, and there have been a lot of variants of this iconic sports car over the years. The company currently lists 21 different models of 911 for sale in the U.S. As an investment, though, the various models sold here from 1999–2005 (designated 996) might make a good bet. These models were criticized for their styling and for Porsche's switch from air- to water-cooled engines, but they'll blast to 60 mph in less than 5 seconds, top out at about 170 mph and handle precisely.

"There's no such thing as a hopeless Porsche," Hagerty said. "Porsche has appreciated more than any other manufacturer in the last three years. Any 911 that people can afford is a good bet, so the 996 is a deal."

Average price for Porsche 911: Hagerty does not publish prices for the 996, but auto site Edmunds lists 1999 year model 911s for sale with prices clustered just above $20,000 for sales within 500 miles of Chicago, Houston, Los Angeles and New York.

This stylish 2+2 sport coupe handled as beautifully as it looked, and featured four-wheel disc brakes and a top speed of about 120 mph. The Alfa Romeo was produced in Italy in a variety of configurations (and names) from 1964–1977.

"You see a huge appreciation in prices for early Porsche 911s, and the GTV is every bit as good to drive as those, so I expect to see these climb in price as well," said RM Sotheby's car specialist David Swig.

Average price for a 1969 1750 GTV: $26,407

This sporty grand tourer was styled by legendary British designer Ian Callum and sold as a coupe and convertible with V12 and V6 engines. About 7,000 were built overall between 1994 and 2004, far more than any Aston Martin that preceded it.

"They weren't considered as special as earlier Astons, and many people have trouble telling them apart from the cheaper, Jaguar XK, which is built on the same platform," said Hagerty. "Still, these are great-looking cars and can be driven regularly." He noted that the DB7 market bottomed out in early 2014 but has started to rise.

If you can't swing the $30,000-plus that a DB7 now commands, Hagerty noted that the similar, Jaguar XK series from 1996–2006 are available for less than half its price. It's slightly larger and slower, but still quite sporty, and with more than 90,000 produced, finding a well-kept example should not be difficult.

Average price of a 2002 Aston Martin DB7 convertible: $40,120

BMW's M3 is the standard by which all modern sport coupes are judged, and all three experts cited the first M3 (designated E30) as a good investment. Even after three decades, the M3 combines spectacular performance with everyday drivability. At a time when emissions equipment left many sporty cars struggling to reach 60 mph in less than 8 seconds, the M3 did the 0–60 sprint in 6.9 seconds. Racing variants were considerably faster and dominated the track.

"They made more than 16,000 of these overall, but not that many have survived without being wrecked or putting on 300,000 miles," Swig said. "We recommend collectors buy the best-preserved example they can afford. Actually, that's true for all collectibles, whether the car is $5,000 or $5 million."

Average price for a 1988 BMW M3: $48,004

This hot hatchback is one of the most memorable, improbable cars of the 1980s. Renault created the Turbo by highly modifying its lowly R5 (sold in the U.S. as the Renault Le Car) from a front-wheel-drive economy car into a rear-wheel-drive racer by cramming an engine where the backseat used to be. And what an engine!

The 1.4-liter four cylinder pumped out more horsepower than many V8s of its day, and in a lightweight body — significantly smaller than a modern Mini Cooper — the car was explosively quick. A 5 Turbo won the Monte Carlo Rally in 1981.

Swig is a big fan, and Hagerty noted that prices have jumped in the past year. An R5 Turbo worth less than $40,000 in 2014 is worth almost twice that today. A simplified, slightly heavier variant called the R5 Turbo 2 is also quite collectible.

Average price for a 1986 R5 Turbo Renault: $78,436

This lightweight racing legend featured a body designed by Bertone and a potent V6 from Ferrari. It won the World Rally Championship in 1974, 1975 and 1976. Less than 500 race and street versions were produced, in total.

"There's a very limited number, and rally enthusiasts hold them in high regard, so demand greatly exceeds supply," Swig said.

While street versions are selling for less than half a million, ones with an FIA (Fédération Internationale de l'Automobile) Group 4 race history can sell for far more.

Average price for a 1975 Lancia Stratos HF: $452,535

HAGI founder Hatlapa is not alone in calling the Countach a "revolutionary supercar," and its average price compared with some other collectible cars from Europe supports that argument.

Launched in 1974 with relatively clean but dramatic styling and 370 horsepower, the Countach got more flamboyant over the years as the Italian manufacturer added wings and strakes to the exterior, and larger, more powerful V12s under the rear engine cover. Lamborghini sold more than 2,000 Countachs over its 17-year run, and many, many more posters.

"Countachs struggled to crack $100,000 for a long time, but have increased in value drastically over the last three years as the demographics of the market has changed," Swig said. "The kids who had their pictures on the wall have grown up — it's the natural progression of the market."

Average price for a 1982 Lamborghini Countach LP500: $462,760