Some people get hung up on thinking that a lower mileage car means a better car..this is not strictly true especially with the Williams. If i was buying a williams again I would go for a AA/RAC car check to make sure the car is in good working order...before doing this however you should pay close attention to the following parts of the car..

Rear arches - prone to bubbling/rust
Inside of tailgate - as above
Wheels - Almost always corroded or refurbed incorrectly, they should have a silver diamond cut edge to the rim. A good haggling point a refurb will be around 150-200 pounds.
Check Electrics- dash bulbs often go and check leccy mirrors if it a 2 or 3. Windows, particularly passenger side can get sluggish or stop completley.
Suspension - Often worn, budget up 5-600 pounds to refresh to new standard.

I'm sure theres loads more stuff that people will add, this is just the stuff I wish I had paid close attention to when I bought my car!