Hi Cinder
Most countries won't allow an engine swap if the engine is replaced with an older model. That is, if the emission values are worse than you originally had they won't allow it. That's why you don't see many G60's in 6N's, even though these engines are relatively cheap.
The VR6 is unpopular, because of its size and weight. Even though the weight argument has been downplayed by owners of Mk4 VR6's, the general opinion is still that they're heavy, making the car nose-heavy, thus making the car handle like crap. The weight distribution of a mk4 is 65-35 as it is already, it only gets worse with a 6-pot lump.
The 1.8T has been done dozens of times now, as it is by far the most popular swap around. I know of two guys attempting this swap at the moment. Be prepared for a long, long process.
Now, to answer your questions:
1. No, it won't fit on the normal mounts. You'll need to adapt all three points, including the gearbox mount (towards the rear of the car), and the not-yet-there middle mount in the front.
2. Most people use the TT/Mk4 Golf gearbox that comes with the engine. I suppose you could use a different one, but why bother.
3. Brakes are usually G60 items, as they're still 4-stud. If you want 5 stud, try VR6 brakes, or even TT ones if you plan on running 16" wheels only. But, G60 brakes are fine, as they are the cost efficient option. The SEAT Ibiza Cupra had 4-lug Brembo brakes at one point, but they're obviously not cheap.
4. The 1.8T has tremendous tuning potential in a lightweight small package. Try getting an early one (pre-2002), with low miles. Make sure to get the dials, the keys, the computer, and all assorted bits attached to the engine as you'll spend ages gathering these afterwards.
5. A breaker's yard, or an engine specialist.
I haven't done this conversion myself, but this is all pretty reliable info.
Hope it helps
