Hi Richard
Welcome to the forum
I could be wrong (without checking) but I think the 996 C4 is narrow bodied.
The 4S is the same as a Turbo with no Turbo.
997 the S has a power increase over standard C.
Both suffer from engine failures and a bore inspection when you have found the right car is worth the expense.
The other big expense are radiators and condensers. They gather crud underneath from the front vents and that rots them out if not regularly cleaned.
You should budget £2k a year running costs and I guess the all important question is are you buying a daily driver or a hobby car?
996's are great buys the 997's will continue to depreciate.
There are buying guides for the 996 and 997 at £10 each well worth the read.
I would pop along to your local meeting and get a feel for the different models.
Feel free to show us any cars you are interested in and ask any questions.