This thread is full of fail.

90-97(flip ups) is 1st gen, 90-93 NA got the 1.6L which is made for boost, with 9.4:1compression ratio, Factory oil squirters. It came out of the old 323 gtx. It loves to rev and is very peppy. While they have the weak rear end and optional clutch type lsd. The rear end and 5speed are good for about 250hp before there done although can be broken in stock trim. In 92 they added a simple rear brace to the suspion.
The 90-93 cars are also the lightest of the bunch.

A 90 shell with a 94 1.8block 99 head, intexhaust cam swap and you have a 2200lb(full stock body with ac and ps) with 135whp.

The 94-97 1st gen NA got the 1.8 which isnt as peppy got less agressive cams and isnt as responsive. But they make way better power With an intexhaust cam swap it gives the motor new live gaining upto 20whp. They got a larger diff they also got an optional Torsen lsd that is alot stronger then the 1.6l models. They also added better chassie braces, one behind the seats and a U shaped one on the rear diff and crossmember, and a brace up front under the tranny.

Then the 2nd gens NB 99-05 are non pop up cars with the 1.8 which in 99-00 had the best flowing head but you couldnt change the timing. everything else is the same as the other 1.8s. In 2000? they got VTC and the intake butterflys to help with low end power and torque. The optional 6speed is pretty much bullet proof while the 5speed is just like the older ones.

02-04 mazdaspeed were a 1.8L turbo car which in stock trim they were good for 180-190whp, Upgrade the intercooler and tune you can make 220-230hp. They also got the stonger 6speed tranny.

Then the 3rd gen NC 05-present got the 2.0L and they suck the car sucks i hate it.

As for aftermarket support its about the same for all 90-05 cars, from motor to brakes you can get anything you want.