Im looking for turbo FC and wanted a 89-91 but they seem to be rare for under 6k in nice condition, but 86 to 88's seem to be more prevalent. Are there like tons of parts that only fit on s5's or does it really just mean 20 hp less for an s4.

 

If you look at Icemark's sticky on what came with fc's youll see that there are minor diffrences between s4 and s5 rx7's.
If you can find a good priced s5 then id say get that, but youll mostly find 87's and 88's for sale.
Almost all fc owners start modding there cars right after they buy them so the hp diffrence doesent make that much diffrence.
The main diffrences between the 2 series other than the hp is the exterior and the weight of the cars. The s4 tend to be slightly lighter while the s5 are heavier. And lastly most people tend to like the look of the s5 rx7's over the s4.