I have yet to buy a 944 that hasn't needed a couple of grand spending on it to get it the way I want it just after I've bought it. I DO tend to buy decent cars but with higher mileages (because I rack them up so why pay extra to take a low miler and ruin it?) that cost a good £2-£3k less than the top whack cars but with my S2's I have replaced the cams on both, and I would be prepared to bet I'd have had to do that with most of the top-money cars available too.
As far as running costs go, I have done 25k in my cab in the year I've owned it. It was laid up for around 4 months of that time but I think driving it would have done it more good. When I bought it I had the cams and suspension replaced for £2k, including a 48k belt service with all new rollers. I have a friendly ex-OPC guy who does excellent work for £25/hr which helps that figure immensley. I also replaced the gear lever gaiter and some odds and sods of plastic trim that was broken.
Over the year I have spent maybe £400 on servicing, £350 on tyres (a pair of rears in 265/35 18 size, standard ones are much cheaper), £150 on having the cams removed again because a bit of valve spring turned up in the sump plug magnet, but it turned out to be a leftover from an old repair (nice, I really like the idea of valve spring bits in my sump...) and that's it. I have a weeping clutch flexi hose (£60 John?! Jeez, need to see what we can do with Goodridge...) and my oil level warning light comes on most mornings despite the level being fine, so that needs replacing. Also my clock display failed (they just do) but I took the one from my Turbo rather than buy another for now. I needed a new brake light bulb for the MoT at 70p. Probably a grand is about right for my 25k year then, but add £2k for post-purchase stuff (brakes, cams and steering racks have been my problems here).
Of course you'll always have a list amounting to several £k that you 'want' for the car...