Hi,
"These are also old lenses - 50 year+ and condition is important. Focus helicoid dry out, fungus between the elements, scratches etc are common - and you don't want to add to the cost once you bought the lens".
In my little world all second-hand lenses, regardless of the makers, are subject to the same stresses and strains etc of use and abuse. Jupiters can be duds and Summarons can be duds and Canons can be duds. And you can be lucky and find them in mint condition.
Experience tells me that putting them right costs a lot or a little and it has nothing to do with the makers but a lot to do with the repairman. I've had quotes for about 300 pounds for a lens repair and paid 20 pounds for one.
The real difference, imo, is that - for example -Jupiters are under valued and Summarons are over valued. That will be reflected in the price, regardless of condition, of buying one. Alas, with lenses you are taking a gamble because you can't check anything easily apart from the feel of the mechanics. The acid test is a few cassettes of slide film and that is not something people or shops do before selling them. FWIW, I think shops gamble on only one or two being dud and charge accordingly and they know the repairman who will do it for 20 quid!
So I'd say buy with your eyes open, do a thorough test and be prepared to take the lens back or pay for a repair.
Regards, David