For the sake of sound I'd go for a Mk5 R32 
Very over rated car.
They're getting old now and cost a bomb to fix. My colleague has spend nearly £2k on his in the last 12 months on just normal maintenance and his isn't even a high mileage thrashed car. It's a two owner lowish miler with full VW and now specialist history.
Plus you're talking thousands to get any extra power out of it which will in turn just turn it into a luxury toy rather than a daily driver and if anything make it's value drop.
They are just a nice noise, nothing more. They should have put the R36 engine in them.
20k is near mk7 gti money these days 
My new year you could probably pick up a GTI PP up for under £20k with factory warrant intact
Jeez autocorrect went mad there! I hate replying on the phone!
Is the mk7 GTI that much better then a mk6 R?
Ive not driven a mk7. I wonder if Id notice the difference? The other main points are is the interior that much nicer and car more reliable etc.. as I said its a daily for me and the Mrs.
Is the Cloth in the R heated? Its a right pain finding one with leather, so thinking I could buy cloth and buy some leather one ebay and swap over if all the wiring and switches there, would that work?
The chassis on the mk7 is a revelation. The torque on the mk7 GTI makes it quite punchy as standard and ballistic when tuned (bearing in mind it only has a smallish quick spooling turbo).
The cloth on the R isn't heated unless you have winter pack.
An awful lot of R's will have leather, not that an awful lot were actually sold but you get my drift...
Cloth really isn't that bad to live with and if getting a GTI it's all part of the character anyway.
VW leather isn't great (understatement) and the cloth works fine for me.
The later model mk6 you go for the more chance of it having leather as it became standard.