Do you guys think 20mm rear spacers on OEM 17' rim is a bad idea? Too much strain on the bearings ?

No, works fine.
 

15mm front 25mm rear
<img src="http://pictureposter.audiworld.com/12902/wheels_and_spacers_003.jpg">
<img src="http://pictureposter.audiworld.com/12902/wheels_and_spacers_010.jpg">

Re: No, works fine.
 

Where can I score a set? Any special hardware needed?

www.esctuning.com will set you up

Questions!!!
 

I am sorry but my english is not good enough to understand the terms that u use...I am from Greece. Could you tell me what a "spacer" is, why you have to change it and what is the difference after doing that. I would also like to know, what other modifications you can have on a TT. I have read about many things I don't know what the hell they are. Can somebody pleaseeee explain these to me??? Thanks a lot.

You will get a better response if you start a new post with these questions.
 

A spacer is a part that mounts to the hub of the car and offsets the wheel out. You can get many different widths and position the wheels out as little or much as you like.
Technically, it lowers center of gravity since the overall track widens but the most common uses for spacers are to either make the car look more agressive or to push the wheels out to clear custom brake setups.

Go to www.esctuning.com to learn more.