The way i look at it when thinking of a turbo to buy is, pick the max power you will want to run, then find all the turbos that make that much max power and then find a good comprimise between which one comes on boost earliest and cost etc. Also things like low mount, straight bolt on, internally wastegated turbos will be a lot cheaper. As for exhaust, any dump and front should be big enough, high flow cat or no cat will flow enough, and a 3.5 inch catback i would want for 250kw.