You should have no trouble with the interior then, just the door card fixings as i mentioned.
The S2 has a couple of screws around the edges, but the XR ones dont, but they should fit no probs with minor mods as they will be the same shape

It will be much easier using the S2 loom with S2 engine and dash, although you will still need to mod little bits of the loom to fit the Mk3 (Rear number plate lights for instance, Mk3 has 2, Mk4 has 1)

Gearbox sounds good! thats what i want for mine