The best thing you can do is get some good instruction on the day - it will get you round much faster & safer. Getting the basics of lines also makes you more predictable for other drivers.
Donington is a pretty good circuit to start at as there's not a lot to hit, so not too intimidating. It also has one of the best sequences of corners of any track in the UK. There are plenty of places for faster cars to get by. I think it's much better than somewhere like Bedford, which is dull and featureless, where the lines are actually quite hard to pick out.
You can usually pick up an evening track day at Donington during the summer. Opentrack are a decent organiser & they provide some tuition included in the cost.
Any track is hard on the car - brakes, suspension, cooling. Donington has a couple of big braking points. You'll also find the car a bit soft, but even so I think it should behave pretty well.