Buying a car in the 2.5k range

So my first car a 1.9 sdi vento(a hand me down) gave up on me last week. So i was wondering if anyone here could steer me in the right direction for buying a car for the first time.

My budget is 2,500 maximum and im hoping to get a 1.9 tdi due to the insurance. Im also going to be driving on a motorway long distance twice a week for the next year so something with around 140,000 miles tops and something with a high mpg would be great.

I had a look at a few A4's, passats and Boras on done deal but some of them seem too good to be true.

I'm mainly wondering if i could get a car in that bracket with the budget i have or if im asking for too much. Also should i trust done deal or should i stick to garages and dealerships?

Thanks in advance for any help you can offer me.