The buyers guide Richard has linked is a great guide for buying. Make sure you look through the history and have a good read of it, as the OPC service paperwork will give engineers reports on any potential faults or issues at the service intervals, and you can see if authorisation was given to carry out the repairs and if they were then completed during that or later services.
Don't be affraid to take time looking at the history when looking at the car, the owner should expect it. Make a note of where the services were carried out, if they were specialists and not OPC, then give them a call with the reg and ask them to validate the work carried out. If there was a recent service call the garage that serviced the car and try to speak to the tech that dealt with it to see if he can tell you anything else about the car (he may recall it if it was recent). If any history is missing, call any garages that are documented to have dealt with the car and ask them when, and at what mileages the car was seem, they will have a record. Always check the reg / vin against the history.
You can get peter morgan to do a check, or an OPC to do a (200 point I think) check for around 150.00 or a more thorough check for around 200.00 (but you will need to get the car to the OPC, and likely the owner won't let you take it there, so you will need to be patient and arrange it around them).
As Richard said, the mileage you are doing will mean you will want to ideally get a lower mileage model. I bought mine as what I thought was a bit of a bargain, it was high mileage, but it was mint, as an example I have only done 750 miles in the past 5 months, so I am in a different situation. You will likely pay a premium for a lower mileage model (although I recently spoke to someone who purchased a C4, same age as mine with half of the mileage for the same price, he happened to be in the right place at the right time!).
Don't rush into buying one - I did (it's easy to when you see one in the flesh that looks, sounds and goes well), and in all honesty I love my 996 (I loved the colour, interior, sound and everything about the car when I first saw it), but my funds were limited, and I am starting to have to add things to mine to get it to where I want to be. Now I suppose this isn't a bad thing, as the I am renewing a lot of stuff vs it being old, however I probably would have rather waited a few months until now when I was in a better position and just put a extra few k in and got a model already with everything I wanted, or got a lower mileage model. Still, who knows what's around the corner (could have just as easily been in a worse position). I wouldn't say mileage is everything though, I would generally much rather have a car with comprehensive history that has wanted for nothing than have a lower mileage model with sketchy history.
Take it for a good, long test as well - several times if you can. I took my car out three times over the course of a week before commiting (although my mind was pretty much made up after the first drive and as soon as I saw it! I just wanted to make sure it wasn't going to let me down when I handed the cash over).
Good l uck.
