what makes you want a t3? the turbo that's on the car already is pretty good to run 20 lbs of boost all day long. do the right work and u can have around 300 hp with the stock turbo
i don't really see why not. with enough done to the engine... look at the Mk iv 1.8T posts i have and then think about it
now, from what i know, if u do go with a t3, u need to get the exhaust manifold taken care of as u mentioned, and get someone who knows how to program the car for the turbo..... and the turbo for the car...... get a BOV suitable for the turbo. and, will the turbo take oil ? it should, so u'll have to find a way to feed it the oil so it doesn't destroy itself..... then u have to hook up the rest of the exhaust system to the turbo..... and the first part of the intake system to the turbo, which u will probably want to modify anyways.