Hello mate welcome to S.O.C
http://uk.subaruownersclub.com/forums/topic/1619-help-check-guide-buying-1st-subaru/
bit of reading here about the cambelt, look at it as a sell by date though. It doesn't mean it will self destruct but need to be changed / eaten sooner than later :)
Nur spec exhaust my favourite but lots on the market depending on cost I guess but never forget about the 2nd hand market :D
Power few place you can start dropping weight is always a cost effective way to get small gains, but if like me not always that easy to make the kids walk, so more in a good filter or induction kit non oiled one otherwise kill your MAF. and decat exhaust
( head back to your last question :D ) Just remember MOT garages done like Decats so make sure you are friends with a garage or can replace one for the mot ;)
Then fuel pumps injectors turbo. I guess be easier to ask how far did you want to go with it :D