I am by no means an expert here more like a genuine motor rebuild newbie. However, my first Porsche (bought by me around '97) was an '83 SC 110,000k on the clock. I can tell you from personal experience with my car that the SC does seem to be bulletproof. I did virtually nothing mechanical and drove this as a daily driver. In my work at the time I was traveling across the country and took the SC with me. This little guy got me from California to the midwest, through the Detroit winter, on to the muggy south in Atlanta, back to California and all points in between without so much as a hicup. After this car I adopted the bulletproof mantra and have always said I would take any SC on a road trip as long as it started, ran smooth and had oil. In fact I blame the supreme reliability of this engine for getting me in my current predicament with one '91 C2 Turbo. I basically bought the Turbo after one real quick test drive. Now I am doing a motor. Anyway thats a different story over here
1991 911 Turbo 3.3 saga (The Black B@stard)
Once again I am no expert but in my humble experience I would say that if the plugs look good, the motor runs smooth, minimal oil consumption, no leaks and most importantly still has the cigarette lighter then you may be able to take her up to 200,000k+ without a problem (unless you plan on racing which isn't clear from your post and I couldn't comment on).
I would call around for a motor rebuild quote and then factor it into the purchase price anyway and see if you would be satisfied paying that much for the car. It seems like he has already done some fairly expensive repairs/mods and I don't think it will cost anywhere near the $15,000 he has invested to do a motor.
Good luck. Your gonna love the SC.
Try Henry at supertec for a quote on motor rebuild. You can also check out a place online called LA Porsche Dismantler and they have used Porsche engines. The 3.0SC seems to run about $1000 for long block to about $4500 for a complete USED motor air cleaner to exhaust tip 72k miles.