My brother has two Euro spec 1983 SC coupes. Both are probably a bit quicker than my US spec 1984 Carrera. The Carrera "comes on the cam" a bit harder over 4000 rpms and the SC's lack that ultimate pull (at least in feel), but his SC's are both quite quick.
Where are you getting the weight increase that you quote? SC's weigh less than 3.2 Carreras if you compare a no option '78 SC to a no option '89 3.2. That's because a lot of equipment became standard as the model years increase. More options typically means more weight. I would suspect that a 1983 911SC weighs about the same as an equally equipped 1984 3.2 Carrera.
My '84 has few options and one of my brother's SC's is very similarly equipped. They both feel the same as far as weight goes. One could feel a 300 pound increase pretty quickly...
Also, keep in mind that Porsche used the 'old school' chain tensioners from 1965 to 1983. So, they are not THAT bad. Keep an eye (or an ear) on them and you could go awhile without trouble. My brother had over 115,000 miles on his SC before switching over to Carrera pressure fed tensioners and those non pressure fed originals were still fine.
I think that an SC, 3.2 Carrera a 964 or a 993 could last an indefinite amount of time if you keep maintaining them. Even a 2.7 liter car with a fresh and properly rebuilt motor can last a very, very long time. The older cars are very robust as well (pre 2.7 liter cars), but some parts are getting hard to find and the older a car gets, the more problems from age that can arise. However, there are several people on these boards that run old 911's in a daily or near daily driving situations with few major problems.
911's are relatively simple cars and are extremely robust vehicles. It's not uncommon to find cars running very well with over 200,000 miles on them.
Good luck,
Jay
90 964, 84 3.2