First, you have to determine if you're building just for track days, or for an IT class. I've seen some "track day" cars at BMW Club events that have modifications done to them that are not allowed for a class-specific build and that would cost a bunch to un-do. So, step one is to get the relevant rule book and build with that in mind.

I think you'd have more options with the E36. In IT, it can run in ITS and ITR (plus whatever BMW CCA Club racing class you could put it in, or even NASA Spec E36 if you don't go too nuts on a build - see my rule book statement above), there's tons of aftermarket support, and it's got that nice multi-link rear suspension vs. the E30 trailing arm setup. It's also almost a decade newer than the E30, in case parts (particularly body parts) availability is an issue in the distant future.

As for resale, for a race car, forget it. The E36 is likely more marketable, but if you look in the classifieds here, you'll find some top-flight E36 cars for sale now for a fraction of what it would cost you to build either car (and a fraction of what they cost their owners to build) - even with your own labor for free. Those cars are also fully sorted, too, so unless the "build" is part of the sport for you (as it was for my first two race cars), I'd sell the ones you have now and buy one already built. Many a racer has gone broke building a "free" car rather than buying an already built one (been there, done that)..