Both cars will get you there, but get them serviced first before taking them anywhere and ask the mechanic who looks at it to point out any problems they spot. Do all the obvious stuff as well - check that the tyres are ok and that you have enough water and oil.
Both cars should be dead easy to get work done on anywhere in Europe.
Small point about the Merc - it's UK-reg - will that mean you need to get it re-registered (and pay registration tax) on it? The motor needs to be 30-years old before the tax falls to a small flat fee... you're looking at a 1978 car, not 1992 if that's the case.
The owner is honest enough to point out the clutch problem with the Volvo - You would want to drive that to see just how bad it is. If it's estimated to have 1kmiles or less on it, then it would be much more sensible to replace it before it goes wrong. Have you a friend in Germany who could source you a mechanic where you could get it done over there for less?
Nothing wrong with an older car in good order.