If yr pretty competent, mechanics wise, and you got a mate that will help its reasonably straight forward, although very awkward!!
Any 020 box will fit, although the ratios are different on certain ones I believe. Best off trying to source one from an 8v gti really, theres a couple on the bay at the mo.
Starter motor off, clutch cable off, gear linkage off, bottom rear engine mount off, both drive shafts off, a handful of bolts holding the gearbox to the engine out, a bit of jiggery pokery and its out, reverse to put back in
You will need a special splined tool to remove the driveshafts, everything else is pretty standard tooling.
A garage will probably charge you around £150-£200 to swap it over
