The 1.8T into Mk3 has been done, so you could find plenty of help and guidance online.
How about something a little more unusual.
A Mk3 with a PD150 engine transplant.
Mk3 looks, power, tunability and economy.
If you were going to do that, then it would be simpler to start with a Mk3 TDI, then at least you don't have to swap fuel tanks and pipes.
If you were going to stick with the 1.8T swap, start with a Mk3 16v, cos the brakes and suspension can at least cope with the initial 150bhp, thus saving time during the swap and do those later.
As you need to keep the car on the road it would make sense to do it:
1) Buy Mk3 16v - drive till you've sourced a 1.8t lump (get everything you need so you're all ready to go)
2) Swap over engine, leave brakes and suspension, get car back on road
3) Drive for a little longer till you're ready for the brake swap
4) Upgrade brakes
5) Upgrade suspension
6) Remap and tune 1.8T engine