Don't get a Nissan Mainstealer to do it, go to a specialist, I take mine to Abbey Motorsport in Oxted from Leicester.
If it's already been done, there should be some paperwork from a previous owner.
I just notice you mentioned an exhaust, is that with stock CATs? I'm pretty sure if you've gone with HFCs or test pipes, you're supposed to get a remap as well for it to run right?