With your budget and the age of cars you are looking at, I'd save for a bit longer and get a Mk6.
I was in a similar position to you this time last year, looking at late model 170 GT Sports, but in the end decided to work a bit harder, save a bit longer and get the Mk6. You'll struggle to find a cheap GTD, but there are hundreds of 140 GT TDI's to choose from.