Hello Tim, and welcome.
Q2, is £260 for the parts, and about £300 for fitting - because it requires dropping the front subframe and dismantling the front suspension. Other than that, its a straightforward Diff swap.
However, at the same time, get the drop links (£10.50 each side, so £21) changed, and either get some new ARB's (Eibach, about £250? front and rear) or some replacement standard ARB bushes (£24 per pair).
Not sure on the cost of a cambelt change, sorry, no idea whats involved in it - can anyone else advise?
If its done over 50k you *may* want to look at getting the clutch replaced at the same time as the Q2, it shouldn't add much to the labour cost but will save some money on doing it on its own as its in the same 'work area' so to speak.
Also, if you going to be doing track work, I'd also advise buying a GTA with the 330mm brakes fittied (or budget for the work, see the pinned thread for costs) and also thinking about coilovers too. They're both considered pretty much essential for enthusiastic driving.
Well, I think that's about £3000 of your hard-earned spent already
Good luck in the hunt, they really are great cars...
*Other owners - feel free to adjust my figures, most are guesstimate, others are EBSpares prices....