Sounds like a lot of the value in the car is within the powertrain. Nothing wrong with that as long as it is the way you want to go.
I had a 72 Chevelle. Paid $2500 for it about 10 years ago from a little old lady. The car was pretty much all original 350/auto car. I had dreams of building up a 427BB (origin of my username) but decided to concentrate on the body and interior first. When I first got it, it was nearly rust free. Just one little spot on the lower fender. But after a few WV winters I started getting rot around the rear wheel well. It was not a bunch, but because the inner wheel wheels lip was rusted too I had to replace both full quarters and outer wheel wheel halfs. Even starting with a pretty solid body, paint and body work set me back right around $5000 (that includes a cowl hood).
I ended up keep it for another 3 years or so after the paint was done but I never drove it. One day I started to calculate roughly how much it was going to cost for my powertrain and suspension mods that I wanted. I figured I could easily drop another $15K. I decided it was not worth putting that much more into and decided to sell it. IIRC I got about $7800. I thought it was worth $10K because of the shape of the paint and interior but I was wrong. When dealing with this stuff, I found out it is gold when your looking for it, but garbage when you go to sell it.
I guess I recommend adding up what you want to do with the car first. Sure $200 here and there is not much but it adds up in a hurry. Especially the detail pieces. My lord it can get expensive.
My 72 Chevelle SS Clone