Due to the notion "you never know what you're going to get," I would strongly consider rebuilding your gearbox than just buying a used one. Not only will you know exactly what you're going to get back, it will likely work better than new. This is also a great time to consider swapping out the factory open diff for some form of limited slip.
As to the difficulty of the swap, it's not too bad. Rear bumper removed, exhaust removed, then pull the gearbox. I'd swap the clutch, replace the RMS seal, and do an IMS upgrade while you're in there, have a good look at your CV joints, then put it all back together and enjoy life for a while
