pantera,
I know that the tensioner thing is probably the first upgrade. with my SC people were always telling me about the tensioner upgrade. I never did it but I could have just been lucky. There is some good info on buying 911 on this site in the 911 FAQ.
911 FAQ
Specifically it says the following regarding chain tensioners
"A2. CHAIN TENSIONERS -- The timing chain is held taught by chain tensioners. The early (pre-1984) timing chain tensioners tended to wear out and suddenly collapse causing your pistons to come in violent contact with your valves. This gets expensive. The 1984 and later cars used oil-fed tensioners which solved the problem.
Many early 911 owners have had their older chain tensioners replaced with Carrera tensioners. This modification costs around $300-$400 plus labor. Adding carrera tensioners is extremely expensive on pre-1967 engines.
Some have modified their tensioners by adding mechanical tensioner guards. These devices keep the tensioners from collapsing completely when they collapse."