Be careful about going for large cars on low insurance groups - they often come with small, under-powered engines that offer as much (if not less) overtaking ability as my first car, a 1 litre Nissan Micra. Such cars won't have the 'modern' small turbo-petrol engines. A 1.4 Ford petrol engine from the late 90s/early 2000s will be about 80-90 bhp. Better to go for a (lighter) Fiesta with the 1.25 ltr engine (or similar powered one from other makes) which is a bit nippier, cheaper on fuel and cheaper still on insurance.

If, as is likely, you are buying whatever you choose at an independent dealer or a PX that a main dealer wants to shift quickly, bring along someone with extensive mechanical knowledge about cars to inspect the one you're looking at (sometimes its worth paying for an RAC person or similar) so you don't end up with a banger that'll break down on the way home.

As others have said, condition (mechanical/electrical as well as physical) and reliability are the things to look for (other than general needs such as size, driving position, minimum safety criteria and annual mileage) - you can also get more for your money in that regard (say a newer model) if you pare back the gadgets and gizmos like A/C, sunroofs and even electric windows to the barest mimimum you need (not that you'd like to have).

Also be wary of buying from rarely-used shopping cars, as they can deteriorate more by just sitting around than those that have done a decent mileage and/or (like my Mazda3) not a high mileage overall (50k miles in 9 years) but a decent amount of that on longer runs. Given the prevelance today of speed humps, shopping cars get much more suspension wear and tear than those used for general purposes (my parents' Fiesta being about 2-3 years younger, less mileage but having to have new suspension bushes three years before mine did). A well-care-for example (make sure you see the service book, and if possible [if you buy privately] bills showing work carried out) but doing over 100,000 miles may be in better shape than a grannymobile that has done just 20,000 over the same time. Sometimes rarely-used cars have warped brake discs which will cost (relative to the price paid for the car) a lot.

Best of luck.