Head gasket questions.. M117

[Ben12345]

Well its about time I tackled my head gasket problem....
Car is a Euro '85 W126.037 with M117.963 5.0 engine. 135k miles
I bought the car very cheaply about 18 months ago after it had been overheated. When I first got it, the quantity of steam from the tips was crazy. I retorqued all of the cylinder head bolts to spec, and threw in some bars leaks for good measure, flushed the brown sludge oil out, and i've been driving it ever since! The car does still use water, around a quart every couple of weeks, but the oil remains clean.

Anyway, down to business.... Whats the likelyhood that just one head gasket needs changing, and how do I tell? use a compression tester? I dont think I can read the plugs, I run propane, so no soot.
I'll be doing the work myself, so dont really want to do whats un necessary.
What else should I think about changing while i'm doing the job? Valve seals? I read that the chain guides should be changed... the chain is quiet at the moment.
I know that i'd have to get the head resurfaced.
I've done head gaskets and rebuilt heads before, but only on pushrod engines, so the removal of the timing chain to get the heads off is a bit daunting to me.

Hope someone can point me in the right direction...