928 prices are still falling - £6k should get you a decent early S4 or maybe a higher-mileage later model if you choose carefully (high mileage isn't necessarily a bad thing if the car's been looked-after properly). Or you could get a really nice earlier S or S2 model and still have a couple of thousand spare for any initial costs.
Major jobs to be aware of are when the timing belt and water pump were last changed - generally, most owners get the belt changed every three to four years as a precaution (they don't do high enough mileages to hit the mileage change interval) and get the water pump changed every other timing belt change, but opinions vary!
Also with later automatics there is the chance of Thrust Bearing Failure - put simply, something moves in the tube that connects the engine to the gearbox and it grinds one part of the engine into another
so, ask if the person you're buying from is aware of this possibility and gets the flexplate checked as part of the annual service and you should be OK - if you get a blank look when mentioning this then get it inspected.
Other common things that might need attention are the fuel tank cradle and fuel lines in the rear wheel arch (as this is one of the most common places for rust to strike, along with the fuel filler cover flap), the ignition leads (if the car's still got its 20+ year old originals they might want replacing and a new set isn't particularly cheap) the MAF which also tends to age (although they can be rebuilt, see
http://www.jdsporsche.com/) and general electrical gremlins, none of which might affect the running of the car but may prove costly to deal with. Also don't expect to find many with working air conditioning!
Basically, if you find one where most of these have already been attended-to you could save yourself a pile of future bills.
Don't let this put you off, 928s are pretty tough cars, it's just that their age is beginning to catch up with them.
If you haven't already found it, it's worth checking out
http://www.928.org.uk for the buyer's guide (including some idea of current prices), cars for sale (there's one S4 in your price range, but not much detail) and there's also a mailing list to keep in touch with other owners and seek help from the more knowledgeable - or you could post a "Wanted" and see if it results in any offers from those thinking of selling who haven't got around to advertising yet.
Finally, if you're not doing anything better Friday week (the 13th
) there will be quite a few 928s and their owners at Goodwood circuit - I'm sure if you checked with the organiser it would be fine to turn up and spectate and chat to people. If you introduced yourself on the mailing list beforehand you might even get offered some rides.