OK. To understand the driving factors here, first you have to know what I want- so here are the big ones:
- RWD
- Relatively cheap to purchase
- Relatively cheap consumables
- Able to compete for 10+ yrs with the same platform (maybe new shell here and there, etc.)
- Decently robust
- Decent suspension travel
What I'm trying to do here is set myself up to have a reasonably competitive car that I can just drive the piss out of and not constantly be chasing issues like I did (and still do) on the Merkur. If I have to replace an engine here, a suspension link there, no big deal as long as it's all somewhat consistent.
Here are other platforms I looked at before winding up on the FRS/BRZ/86:
RX8- Cheap, good amount of interior space, front suspension geometry not terrible. But the rear suspension is a short travel disaster, and the suspension attachments to the body are all tiny and flimsy, and the brakes are too big to fit in a 15" wheel, and I honestly think the supply of these cars will dry up in 5 years once all the cheap ones have finally been scrapped.
S197 Mustang- Cheap, good amount of interior space, simple suspension design which is all steel and relatively easy to reinforce, looks like V6 brakes can fit a 15" wheel with some experimentation. Ultimately I decided not to go this route because heavy and (relatively) high powered cars eat tires and break components, I still think it could be a viable option if you have a large tire budget or aren't feeling particularly competitive.
Earlier Mustangs- Lighter than the S197 but garbage suspension geometry and they're getting harder to find in reasonable shape.
FB or FC RX7- Another really strong contender but they're short on interior space and getting harder to find parts for.
350/370Z- Cheap, drivetrain seems pretty strong, but oh god the suspension. Yeah, you can replace stuff with steel but take a look at the front uprights with that crazy long swan neck thing and tell me you trust that to land a jump a couple dozen times.
Genesis- See previous entry.
318ti- I thought pretty hard about this but when it comes down to it, I just don't like BMWs very much. This is an actual good option that I skipped just because of personal preference.
Other BMW platforms- Dumb when the 318ti exists, since e30s are getting more expensive and the newer stuff suffers from the same weight and suspension problems as most other newer things.
F-Body, GTO, Other Muscle Car Platforms not mentioned- Heavy. Really, really heavy.
Various trucks- Potentially a good option, especially if you're happy not being competitive and running A/Ts instead of rally tires. Not what I want right now.
Metric Chassis GM- Another one I actually really considered, since the dirt track world has developed every component imaginable for these. Same consumables problem as the other V8 stuff though, and probably not really what I'd enjoy driving.
Cool Older Small Things- Celica, Corolla, Datsun, etc. The problem here is that, while the formula for prepping this general type of car is pretty well spelled out by the many rally Escorts in the world, it's all going to be custom stuff grafted to a shell that will only get harder and more expensive to replace. Still cool, maybe some day, but when it comes down to it I want to rally a lot, and rallying an awesome car less is... less.
MR2- I like these, but not for stage rally. It's hard to fit in one with a cage, and the only competitive one I've seen crashed a LOT.
XR4Ti- I don't really like driving mine, why would I want another?
Volvo 240- These used to be "the answer" but I really think they're going to be very hard to find in a few years.
These are just what immediately comes to mind, but I've been chewing on this question for years so if there's another car you're looking at feel free to ask about it.