I think the extra £3500 for the GTD is worth it.
At the moment I am sticking with it.
By the way warranties on second hand cars are a pain, they don't cover what you expect and take weeks to sort out. Given the choice I have always gone for no warranty, its easier to fix it myself. (haven't found anything I cant sort out myself in 24 years of driving yet)
The difference in figs is primarily down to interest for the GTD that you won't be paying on the GTI, doing your own servicing and disregarding fuel savings on the fuel that your company pays for. If you have £10k available, have you considered using the whole £10k and getting a £15k bank loan at 5% or thereabouts (assuming your credit rating is good to very good) over 5 years, and then if you change cars at 3 years (and take out a new loan at that point) you will have more equity in the car and will have paid less interest as a result.
I used to do this prior to the last Scirocco going through VW finance, as I find it difficult to find a bank loan at a good rate with more than £15k borrowed.
On your proposed terms, you will be borrowing £18k for a £25k car (£7k down), and you will owe £13500 at the end of it. So for your £236 a month, you are only clearing £4500 of capital, which is £125 a month over 36 months - the other £111 a month is interest. This is how the high residual value pays dividends for VW finance - you will always be paying interest on the majority of the car's value, from month 1 to 36.
If you can get a £15k loan at 5%, you could end up taking it out over 60 months at approx £283 a month, and settling at 36 months. You will have overpaid by £47 a month vs VWs terms, but at that point, you will owe just over £6500 on it (including 2 months penalty interest for early settlement), vs £13500 GFV on the VW finance. £47 a month paid x 36 months = £1692. So in effect you are overpaying by £4692 (inc your other £3k: £10k down vs £7k down with VW) against £7k difference between owings at 36 months. There's a £2300 saving to be had with the personal loan route.
If you were to go for the GTI, the one thing I would insist on getting done would be the timing belt (or was it a chain back then? Thought the chain was brought in for MK6) and water pump unless it was done quite recently and fully documented as having been done. Apart from that and the cost of the turbo or gearbox, anything else that could go wrong is relatively cheap to repair/replace.