The w124 is by far one of the nicest cars i own, and i too am looking at an additional model to purchase,
if power is no issue, the 220 will suffice, and will be more than adequate on long distance drives, however, in town, it will struggle...
checks from my own experience...
arches, light joints and hinges...
check window movement and the hood... sometimes they can stick on first try, no big issue, as on a 14 - 15 yr old car, it is just a wear and tear aspect, my motors are re greased every year, and it should keep them going for ever...
in terms of interior, just surface wear and tear is the only thing to look for... the internals will almost always be solid..
check for the points where the lights meet the metal, and these sometimes can have rust issues, and if this be the case, could be a sign of possible accident damage or atleast previous cracked light units...
wheel bearings and suspension should all be fine, but look out for smooth rides..
the wheel bushes have alot of weight to hold up, and a bigger after market alloy can shift the balance points, and potentially increase wear on these parts.. worth having an AA inspection if you have any deep concerns!
Good luck!