the main reason for buying the mk4 was because of the engine, but i much prefer the look of the mk3 so it's kinda combining the two, i know that its been done before but that just means that if i need help i know that there are people to ask with experience. i know that just buyin a mk4 would be the easier/ sensable choice but i like a chalange lol and also my missus if of to uni for three years so it's kinda like something to keep me busy lol.
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
this sounds like a good plan, i could'nt afford to do it all in one go so this will be the sorta thing i'd have to do