I had a 911 SC for a number of years. A 1978 Euro car (gray market) Rebuilt motor, stock suspension when purchased (in 2000). Spent lots of money "upgrading" the car, but it needed almost no "maintenance" money other than oil changes, valve jobs, and a change of plugs.
Some will say, and rightly so, that a 911 (sc) is a $20K car. Yes, you can spend $5-8K and put 13K into it, or, spend 12K and put another 8K into it, or spend 17K and put only 3k into it, but the bottom line, assuming a good, non-rusty example to begin with, is that it will become a $20K car.
Be wary of:
Pulled head studs - no easy fix here
Chain tensioners - make sure they have carerra tensioners. Cheap part, some labor to install, but much more reliable than the older style pieces
Pop-off valve - with out one you can blow the "airbox" which, is another very exensive failure!
Ethanol fuels - they can accelerate hardeing of already old fuel lines that were not designed to work with ethanol based fuels.
CIS components themselves are getting old. Some parts are harder to find (WUR's for instance)
Motor rebuilds (shop) are $6k-12K depending on what's needed to be done. CIS isn't "tuner" friendly.. so getting power out of them is a bit harder. Stroking the motor to a 3.2 is a nice way, 964 cams help as well. SSI and 2in-2out exhausts help. Could see 220hp on a dyno after these mods. IIRC, the '81 and newer US ones have better (bigger) valves than the early cars, which will help them breathe abit easier.
Doing a full replacement of 25 tear old suspension (shocks, torsion bars, sway bars, bushings, shocks) can be $5k shop bill right there.
That said, there isn't a better "bang for the buck" used Porsche out there. These cars are very fun to drive and, when set up properly, will run with the newer cars (albeit, not without a good driver
)
The SC's have a "915" transmission which is much "notchier" than the Boxster. I never minded mine, since it was an improvement from my 914's transmission. The 1st-2nd synchro will wear is you aren't patient between shifts and no Porsche transmission enjoys being shifted quickly. The 3.2 Carerra ('84-'89) also used the 915 for part of its life. By 1987 Porsche intoduced the G50 trans, which was hydraulic and smoother in operation than the 915.
Not sure what your budget is, but an early 3.2 Carerra is very similar to the SCs. A bit more HP/TQ but also a bit more weight. You might consider these cars in your search as well.
If you are NOT SC knowledgeable, find some people/shops that are and have them look over your potential purchase.