first thing you have to work out is for a road car will the engineers approve of the mod, talk to them and work with them b4 you start any work,
comparing rotary with 4 cylinder engine,
piston engine is set up to last, when running big power out of these they need rebuilding about every 2000ks same with the rotary,
but if its being set up as a daily driver you want reliability, so 150000 + ks piston engine are good for this
the RX8 is better on fuel then XR6 T, but they are reliability designed cars if you are after power that all changes
so go for it, but im not sure you will be able to keep under your 15k limit going in a FWD as a lot more things need to be changed in order to make the rotary fit, where a RWD theres less changes need made
well thats my 2bobs worth