That's a good quote
I'd do it. It would be very easy to do, if you're handy with a spanner. The exaust off a GTT will do fine, as James says, also the gearbox will be fine. You would want to keep the gearbox, so you can keep the GTT clocks (as the speedo pickup is mechanical on the campus, and electric on the GTT) and the subframe on the campus and GTT are the same, just the GTT has a gearbox stabiliser (sp), which is just bolted on anyway.
My dad and me have removed / swapped a few engines from 5's and I can honestly say it's not a killer job
What I would do TBH, is buy your GTT as you plan, then buy a whole campus (maybe a runner but failed MOT on rust / suspension or something?) for like.. £150, then you have everything you need, and also, you can use it as a reference point (i.e, checking whether bits and bobs are the same, or if you need something you had totally forgotten about), then when you're done, strip the campus for spares. I did something similar and ended up making a proffit, even though I kept the engine!
Wow.. that was an essay.
PM me if you need any advice / help.