I've not owned one, but was going to buy one back in 2006 so can give my opinions on them from way back when.
I would personally only look at the 1.8T 225 Quattro, or the 3.2 V6. My experiences of driving a few 225s were that didn't seem that fast, or handle that great. But they look great in my opinion, really nice design, much better than the mk2 ones in my mind. I thought the interiors were nice, and quirky in a good way, and in the right colour combinations still look prestige.
I was looking at a silver one that was a couple of years old, and it had some signs of rust on the door sill (where someone had shut the seatbelt in the door), and I also tried to buy a blue one with cream interior that was really really nice, but it didn't have bose like advertised.
I think they look great in more bold colours, most of them sold seem to be black and silver though. The roadsters always seem to be cheaper for some reason, but I think the coupes look better.
Check out the ttforum for buyers guides, I must admit I've been put off of them after reading about some of the bills people were getting. They don't score well on reliability index, and have some known issues with the engine (mostly censor based), and the well documented dash pod too (which is less of a problem now third parties can repair them).
Very pretty cars, and looking very good value for money, but I would strongly advise you get one that has a stack of history with it, and ideally one that has had a suspension refresh recently to cover that bill off.
They have the curse (maybe a little harsh) of being designed rather than engineered, so things are packaged a little strangely, and can be a little hard to work on with some jobs. Much like the A2, which I see as a very similar design.
Not sure if budget can stretch to it, but look out for the Quattro Sport models which were strict 2 seaters, and featured a rear strut brace in the back, and a black painted roof, as well as some other bits here and there. You can get a haldex controller for the quattros (1.8T and 3.2) that will split the power 50:50 between front and rear, as they are mostly front biased normally. In the rain, they're probably great stock, but in the dry the Megane 225 Cup I had at the time, would romp all over them, and it felt much more aggressive in power delivery.
That said, I'd buy a mk1 TT, if it was cheap, or really well looked after. Aim for an enthusiast owned one from the forum (plus you get to read back through their posts to see if there are any issues
)