Hi, for £20k you're looking at the top end of 964's, if you've never been in the motor trade, and are not sure what to look out for, get a professional inspection, it's only a couple of hundred quid and is a small percentage of your budget, it will pay for itself as it will show up potential problems, which will give you leverage on the price. Also an inspection needs to be by a Porsche specialist!
I'd recommend the Peter Morgan "ultimate buyers guide" book, it will explain what to look for, don't be in a hurry to get one , take your time, I looked at half a dozen all over the UK before buying mine. It's very easy to let emotions get in the way and buy the first one you see.
Good luck