blackvenom

Im pretty sure my gearbox is in the way out on my "00" 2000 uk turbo classic. I'm on the eye out for a replacement, would I be Better with a sti gearbox and marching diff? Or maybe a gearbox from a newage wrx? I don't have funds for a 6speed conversion. My current box would have 70,000 miles and I know it's had a easy life. Also how much would I expect to pay to have a gearbox fitted?