That's not entirely true. Some parts are just difficult/impossible to find. Some parts aren't shared with other cars (the Impulse shares its parts with several Isuzus, Geos and others). Some parts are just plain expensive - as you mention - so even if you can find them some people just can't afford them.
Being able to find parts for one rare car doesn't mean that it's equally easy to find them for another...
Most of the parts for my car can be gotten from a Geo Storm (Isuzu Impulse Nero). However, the driveshaft, transmission, differential, rear axles, and rear hubs are unique to the RS model and simply do not exist as replacement parts.
There are solutions for all these problems except for the rear hubs. I had CV Pros remanufacturer one of my axles for $261, same day service too. There are companies that can revulcanize rubber for non-existant parts like the driveshaft center bearings. When the time comes, I will be making new rubber suspension bushings from hockey pucks. Front sway bar end links from a C4 Audi A6 are a direct fit replacement. There is very little that I can go get at an auto parts store (front and rear rotors+pads, front axles, that may be it) but thanks to eBay, web forums and the internet in general, I can find just about anything or a replacement/solution for it in a reasonably short amount of time.
With many older/classic cars, there are aftermarket solutions available for just about everything. You could *probably* build a brand new '69 Camaro starting with nothing more than a donor frame, some auto parts/specialty company catalogs and a pile of cash. Same for '64/'65 Mustangs.
TL;DR - There are solutions available for nearly any problem you could run across owning an uncommon car.
To stay on topic and answer the OP's question, my personal choice for it's combination of potential performance, looks, aftermarket support, lack of FWD/automatic, and affordability would be a 1G DSM. In accordance with my automotive tastes, it would be an odd/lesser known one like a Plymouth Laser RS Turbo or Eagle Talon TSi.