Right lets see if we can channel our resources and knowledge and help some of the new members,and those to be, what to try and look for when buying a Williams.
It should just consist of straight forward facts. If your looking for the differences between them then there is a sticky for that.
Anyone in the trade your comments and experience is very welcome.

Here is my experience..

So, Ive gone back to the future, and i'm on the hunt for my all time favorite car. I'm looking at adds on the net, found this club here also, and me mates are spotting them for sale left right and center. I know from the outset that no matter which one I buy I'm gonna end up throwing money at it. How much depends on how much fun your having or want to have!

Lets start with the mileage.
These cars are circa 16 years old. Average mileage per annum on a family car is 12,000 miles. So that would make anything with less than 196K on the clock, below average mileage.

Owners.
Some people are more fussier than others but just because it has had 10 owners or more does not mean it is a bad car. Far from it! It could well have been that the cost of maintenance was spread out throughout all the owners, which is a good thing in my book, rather than have say 3 owners that did nothing but oil changes.

Bodywork.
Easily and without argument the most important element of buying your soon to be old classic. Their French, and they rust badly whether you look after them or not.
IT IS the most expensive and requires the most skilled of pro's to restore the car back to showroom condition (thats if you want it to look, shall we say mint). Look inside the engine bay at the sills, left and right, also get underneath the car if you can and check out the sills. Anything with a sunroof and watch out there is no water puddles in the cabin,or even signs of wet carpet as the rubber around the rear windows do leak on some.
As for the speedlines, get them refurbed. But for the love of god make sure you get the color right!

If your not overly fussy about the exterior look, then just watch out for badly corroded parts such as the sub-frame, rear axle, brake calipers and lines and the exhaust. These from my experience were the most expensive to clean/replace.

Suspension.
Anything with mileage over 80K will probably need suspension renewing. That means shocks(front/rear),top mounts and arb bush's, possibly as well the bottom ball joints and wishbone bush's.
Steering rack track rod ends will need looking after, aswell as the play issue (on the linkage) with the steering rack itself. Also bearings are a bugger to do without a press so bare that in mind also.

Engine.
IDEALLY you should bring along an experienced mechanic or someone you trust knows their reno's,and who can run checks in this area.
But you can assume that if the service history does NOT indicate a timing belt change,water pump change,aux belt change, clutch plate/cable service, coolant flush then these items will need to be carried out ASAP.

Interior.
Make sure your happy with the overall condition of the cabin. Seats,dashboard's even carpets can be found ten a penny.
Door cards, parcel shelf and gearstick leather surround not so much.

Bottomline
So I'm back from the future, and it's been 6 years since I bought my Williams. Purchased new for £2K. Spent £6K.
Condition wise it's as good as I can get it with the resources I have at my disposal today.

Thinking with my head, if I was buying my first Williams today I would easily be prepared to spend £8K on a perfect example, regardless of phase. (that is without the need for getting the spanners out, ever again)
Otherwise, i'd buy a fully loaded track day car.