Thinking of 2.3T in an RX7

Complete 86-88 Turbo Coupe motor, EEC, etc. You can't go wrong. The 87-88 TC's have a Warner-Ishi (IHI) turbo. It's small, but spools almost instantly. It'd work great to get started and get the bugs worked out. If you want to stay on the cheap side, later you could move to the Garrett T3 that the SVO's and pre-'87 TC's had. For some real fun, there's always a Holset HY-35 from a Dodge Cummins Diesel.

Stay away from the non-turbo heads. The chambers are too tight for forced induction use. The downside to that is they're much more plentiful than their turbo counterparts who are prone to cracking.

As far as reciprocating assembly, the only difference is the pistons. Turbo motors had forged pistons with a slight dish, the non-turbo stuff came with cast flat tops. Cranks and rods are the same.

Blocks are blocks for the most part. No mexican miracle blocks here. The two major differences involve turbo oil return plumbing, and main journal size. The '91 and later Mustang blocks used a smaller main journal and are machined for a crank position sensor. No big deal really, you'll just need to figure out the oil return path for the turbo. All else is basically the same with minor differences that don't amount to anything that could be a deal breaker.