That's like asking how long is a piece of string. As long as it has a full service history, the features you want, and is affordable, then take your pick. Naturally you will only get what you pay for, so the higher mileage cars will generally be cheaper, however a carefully driven and well maintained 100000 mile car will be better than a 50000 mile car that has been thrashed by a boy racer. That's why condition and service history are so important. If you are not mechanically minded, take somebody with you who is when you look at possible purchases.
Let me add two things to the above. [1] Don't be sucked into any dealer supplied extended warranty. They are all worthless. [2] Bear in mind the big changes to car tax that are coming in April.