Theres a mass of history about the Williams that I used to write about all the time but have lost the will a bit since I sold No.123 a few months back.
I really cant recommend any other hot hatch over the Willy in terms of driving. Whereas most hot hatches are talked about in terms of each other, the Willy seems to get a lot of comparison with other driving tools such as the Elise, Scooby, Delta Integrale etc. I dont think its too rash to say that its probably the most driver-orientated hot hatch of all. To think that this is over 10 years after its inception makes it even more remarkable.
Its fairly easy to buy a very average Williams for high money. I suppose different people have different ideas of what "mint" means, but 60% of mint Williams are far from it. The best ones are treasured by their owners in a way that most hot hatches never are.
Running costs can vary. Sorted cars that have led a gentle life are often less hassle, but remember that cars wear with age as well as mileage. There will be very few cars that have not had/need near rear arches, stone chip removal and wheel refurbishing. Interestingly, I think that there are very few "common" mechanical problems - although diffs and gearboxes would border this definition. I would always buy a car that has had the cambelt and clutch done if possible, as well as the cosmetics I described. Overall though, the running costs are extremely reasonable compared to the Fiat Coupe 20V Turbo Ive recently moved on to. Some Renault-only parts are extorionate though (mainly electrical switches, sensors etc. - i.e. not the sort of stuff that takes your car off the road).
All in all, the Willy is an excellent buy on a budget. I think you could expect to retain most if not all its value over a year or two. People will always pay for the best and mine sold for a small profit after a year and 10k miles (negated by the running costs unfortunately).
My prime advice would be to hold out for the best and do lots of research. The best examples are out there and do come up from time to time.
Good luck!