brakes, suspension, engine mounts, intercooler, uprated boost hoses, decent filter and obviously uprated dv.
My choice would be a gt28rs, a comfortable 300 with room for more if you ever go as far as upgrading the engine internals. IHI turbos are also a popular choice.. probably because you can pick them up for peanuts from subaru owners who are upgrading to a td06 or something.