My '87 has the (evidently) notorious high idle issue; it can all be attributed to vacuum leaks that I have yet to sort out. Kind of my summer repair plan.
Basically the biggest culprits of idle issues are the Idle Air Control Valve and the 8 rubber intake grommets. Those grommets are what I need to replace this summer.
According to my receipts, the timing chain was 1100 bucks in 2006.
I'm not capable of anything involving opening the engine, as I am deathly afraid of reassembling incorrectly, but if you have/find a sweet indy who specializes like I did, they actually seem to love working on the M117 as opposed to newer engines.
Anyways, I would say go for it; I bought mine from a family member who maintained it ridiculously and it hasn't given me any significant trouble for the past 5 years. I just factor between 1000-2000 dollars a year in maintenance.
I won't get into gasoline costs, of course
Good Luck!