I was in a similar position when I was buying my first house. I had an S3 at the time and had to sell up for some extra cash. In the end I bought an Impreza WRX as it was half teh price of my S3 and only 6 months older. It was a big mistake and it end up costing me more in the long run.
As others have said, sell your car and buy something really cheap to get you about in. Who knows in 6 months, you may get a better job or a payrise and buy something nice again (that's what happened to me!).
That's IF you really are looking for a deposit. It almost looks as though you're just bored of the GTI and want something else. I know it's hard to take a step down in cars, but it is the right choice if you want a decent house. I went from a BMW 330cd convertible last year to my GTI and it was a pretty big step down, but again I got a promotion and bought another BMW as well. If I hadn't downgraded, I would have run up big debts and not been able to afford the new car when I did get it.
