1.8t followed by:
remap
descent disk pad combo
braided brake hoses
brake ducts
better suspension, stock suspension is rubbish
strip it
its a reliable car, there's many things you can bypass on the later models that give you a headache like n249 and n112, put things back to basic like on the AGU. Easy to work on and reliable providing you don't get a complete lemon. It's easy to work on aswell, plenty of tuning options.
I will be replacing the mk4 next year for the mk1 TT and swapping heads over from the golf and doing a full engine rebuild. Stock rod last around the 300mark, valves go at around 350, pistons at 400... so not a bad engine internally for tuning the 1.8T's.