For the best economy and lowest VED (£0 at the moment) the 1.6 TDi 105 bhp Bluemotion is the one to go for and by all accounts is a decent enough drive. A 2010 model with 30k would be around £10,000 from a dealer.

The same money would buy you a 2010 GTD with the bigger 170bhp 2 litre engine, sportier suspension and more extras but much higher miles. A similar mileage car to the 1.6 would cost maybe £2,000 more than the 1.6. Penalty is probably 10 to 15mpg at the pumps (still high 50 mpg+ possible) and a higher VED, and more insurance. Worth it IMO for the extra power, (0 to 60 in 8.1 seconds vs 11.3) better handling (relatively as its a Golf not a Focus) and spec.

Whatever you choose factor in a cambelt and water pump change every 40,000 miles or 4 years for around £400. If you're negotiating on price and you want a bit more out of them you might get some results asking for service or work like this to be done as part of the deal as it costs them less than you'd have to pay but as daveyjp says just ask for the best price with no extras. Ask and then keep quiet the sales people rely on you talking yourself into the car. Do your homework, if you like a car you've seen locally but there's another one cheaper miles away on the internet let them know you're considering going to look at it. If you're not 100% happy walk away and keep looking.

I realise that it isn't PC to say this, but can you get an older male to accompany you ? One of my friends was done well and truly by a garage and it cost her dear. She always took her father with her after that.