well the cheaper, more reliable route would be to save up and buy a turbo two motor/tranny/rear end and save up until you can afford the swap. boosting the n/a s4 motor is in no way gong to be cost effective nor will it be reliable on a budget. auto crossing will definitely allow you to learn the physics of the car and will help you determine whether or not boosting it will be worth it. i have seen some very clean s5 T2 swaps into verts but swaps do cost some $$$.

some things to consider would be to have the engine rebuilt, and ported. in auto cross you would want quick responsive throttle, and with a turbo you will have lag. I'm not dogging your aspirations in any way, just trying to help out. there are four stages to porting in each step higher takes away from daily drive ability. and rotaries are NOT known for their reliability as it is. heat is ALWAYS your biggest enemy. trust me have the engine rebuilt and ported and you will not be disappointed. but thats IMO so good luck with the car and welcome to the world of rotaries.