well right off the bat, several things i can think of.
you need a free flowing exhaust to make max power on a DSM, but they are expensive. make your own downpipe that just points straight at the ground and ends there. you dont need no stinking exhaust
you can use the upper portion of a stock downpipe for this mod. just cut it off about a foot down and have a straight pipe welded on it.
dont use a large turbo..... keep it simple, as spoolup will be important to the autocross event. one of the new dual ballbearing 50 trims would be nice. the cool thing about this event is that you have to have a well rounded car. typically things that do well in autocross dont dragrace well. its awesome that you have to do both. keep that in mind when your choosing your parts.
engine management systems are expensive, but chip tuning is free. get up with a local chip tuning guru to avoid buying an SAFC or LINK system. your gonna need a good race gas tune cause stock boost levels wont give you a chance against higher HP cars like the mustang/camaro.
cams are expensive, but DIY port work is cheap and easy. i can help with that. you can also check on adv/retard the cams to make max power by modifying the stock gears rather than buying new ones.
completely strip the car down to nothing inside and out. no need for anything thats not structural or speed related. get it light, and huge HP numbers wont be as critical.
try to set your self some goals as to how much you want to spend in what areas. the budget is important, cause im sure they will check it out really well.