Pretty much all of us have been in your situation at some point and some good idea's have already been said..
If your going to start modding any engine then it has to be able to breath properly so you need to get rid of any restrictions such as a standard airbox/filter and standard exhaust, so the air flow is maximised.You'll pick up a full stainless for around £60 on Ebay and a filter for about £30.
Turbo wise you can either up the boost a little on the standard T2, great for a standard engine as it spools up quickly. If you want to go that little bit further then yes a T25 would be good, takes a little longer to spool up but it'll stay there for a lttle longer too. When you start putting bigger turbo's on then as advised other mods would be a benifit such as re-jetted carb, alloy intercooler, big down pipe etc...
In my last 5 I ran a modded carb with a 135 jet, front mount intercooler, Ported head, Garrett T25 Hybrid, 285 cam, big down pipe, full stainless and a filter. That was just over 5 years ago but I think technology has come along was since then
it was quick for it's day and regulary ran a 13 second quarter mile with a bit of spin off the line.
Do you have any power figures in mind you would like?