Depends on a few things, make sure you get a clean one to start with. I think the budget of £1800 is realistic and you sound like someone that will look after your first RS
Common sense will see your RS last, regular servicing and oil changes will help but expect the unexpected like with any Ford RS they have a habit of letting you down when your skint lol but luckily if you stick with PF there are so many people with experience of Series 2's that most of your problems will be solved/sorted cheaply as long as you don't mind getting your hands dirty!
As far as looking for problems take a friend for deffo and just check everything spend some time and ask loads of questions to the owner about everything. You can usally tell if the owner gave a shite about the car just by presentation. Make sure you view the RS's in daylight aswell and not petrol station lighting. Also don't travel far unless your 110% sure its going to suit your requirements as I can speak personally of some wasted journies for 'mint' cars that turn out to be shitters, also there are a lot of shitters so don't get disheartened if the first load you see are crap.
Good luck and don't forget to post pics when you buy it