Here's my input starting under $25K. Note--- all of these have to be unmolested, unmodified, clean, original cars. Modified cars (Except for Pro Touring cars) never seem to bring as much as bone-stock cars. Also note--- most of these will never be super expensive....they will just go up a bit. These are all also easy to find....for now:
Lotus Elise ---you might be able to buy one for $25K If it's a non-salvage title I'd expect this price to double within 10 years
IROC-Z , Pontiac GTA---- the 5.7 liter cars will be worth the most. No, they didn't come with manuals, but most collectors don't care
Acura Integra Type R---- a true factory racer. These will bring $50K some day
85-86 Mustang GT--- Signaled the return of the Mustang to high performance, best looking of the Fox-bodied cars. Just try to find an original, clean one
Mk1 Scirocco S /// Mk 1 GTI ---- Never going to make you rich, but good ones are already nearing $10K and will go up a bit
Triumph TR6 --- Last of the "affordable" 6cyl LBCs. Good ones are bringing $15K now, expect that to double within 10 years
C5 Corvette Z06--- Stupendous performance (especially in 405hp form), Corvette cachet, and cheap to keep. Good ones can be found for $15K these days. I'd think 10 years from now they will be worth double that.
107 Chassis Mercedes Benz cars--- especially the early (small bumper) 450SL and last 560SL. Luxurious, iconic, and perhaps the best built cars ever made. They are cheap now.....but look at prices for the earlier "Pagoda" cars.....these 107s are poised to jump....and soon!
There's always more......like the Z3 M cars, the Caddy CTS-V Wagons, and the early Boxster S......but I'm outta time!