Good decision.
If you do a search you will find some good threads on this, none of them that recent though.
Bodywork is becoming a bigger consideration now. IMO it is the most important thing as it is the most expensive to sort out and can be totally uneconomic to sort out if it is bad. Good spots to check include the inside wing around the fuel filler, battery compartment, door pillars, underside. They usually go under the headlamps as well but that is not necessarily a big issue but look for it and budget for it - genuine new wings are pricey. All the usual stuff applies for eg rust, welding, crash damage.
Engine. They usually smoke on start up but that should clear completely and they should idle smoke free. Some oil leaks are minor some are obviously serious. Top end re-builds are very expensive - £4.5k or more plus Vat, bottom end is not as serious if you are to do the top end but again depending on what is needed can add £1k-2k.
Gearbox re-builds are likely to set you back £1.5k to 2k. The good news as Pete says is that the 3.0SC engine is very dependable; the gearbox wears well but at this age if it hasn't had work it will show it - there are some oils that can help. Clutches are not serious but it is still a job that involves getting the engine out - so 5 hours plus parts - and while the engine is out there are bound to be a load of things that you might as well do to make the most if the labour eg renewing seals, sound proofing, changing pipes, heater flaps, heater cable, poss skim the fly wheel.)
Exhaust System - is a lot more expensive than average cars. You want to see stainless steel in good condition for all sections and the heat exchangers and don't let anyone tell you different especially not the seller! New exhaust side and back box will set you back around £680 plus vat fitted, heat exchangers are upwards of £200 each plus vat, parts and fitting. To do the whole lot is around £2k unless you do it yourself - the heat exchangers are best left to the professionals as the studs can be major pain..
There are quite a lot of upgrades that you would regard as a plus - hydraulic chain tensioners are vital as is an air box pop-off valve; others are desirable eg turbo track rods, bilstein shocks, head lights (must be done properly or you will pay with wiring work). See 101 Projects For Your Porsche 911 by Dempsey.
There is a load of stuff that I would term mods rather than upgrades which tend to be more radical and not necessarily a good thing depending on your point of view and what you want to do with the car eg 3.2 carrera seats, duck tail, no tail, bigger brakes, lowered suspension, different wheels (especially if not Fuchs), more suspension work (anti-roll bars, strut brace), sports exhaust (whether they do any good to output and torque is a matter for debate).
If you want to use the car eg touring, daily driving you want the upgrades. If you want to fast road it/ track day / concours it you would have different priorities for originality and mods accordingly .
There are plenty of little things that can go wrong some relatively minor eg rev counter, oil sender, ignition coil, trim, dash lights/ switches, blower motors, heating (can be expensive to put right), wheel bearings, electric mirrors, electric windows. Don't expect the aircon to work. Orignal heater control boxes on later SC's are a bonus if they work as they are too expensive to replace but it can be adapted to manual levers to do the job.
Brakes, tyres, wheels, door hinges, electric motors, more or less are the same deal as you'd expect but perhaps more expensive than the average modern saloon.
There are a few ways to go - personally I'd get a car that has been regularly serviced somewhere reputable , speak to the garage first and get it inspected as well either there or independently as condition can change. There are a couple of tests for the engine - compression and output that can tell you a lot as well. It is a good idea to get a car where the chassis, and engine no's match and are correct and this can be checked with the Club. Also you probably want one that is original RHD or original LHD not changed around.
There are lots of views on it but I reckon you have to budget on at least £11k.
You can join PCGB before buying and that is well worth doing IMO.
BTW, the earlier SC's had an output of 180BHP and the later ones 204BHP.
Good luck.[
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