two very good cars for that purpose, IMHO. The only issue might be cost of parts when they break, and who would be doing the maintainence as the dealers would ream you.
They made the 240 in a diesel for awhile back in the 80's dreadfully slow. I'd probably pick the volvo if I could get a mtx just simply because it would be a newer car. The mercedes could easily get 300k plus out of that engine, the only problem is the engine would outlast the body
EDIT: Saw the MB, that looks like it's insane condition for it's age but those wheels are NASTY!! Anywho, being that you live in mass 4 snowtires would be a must (studded?) Southern cars won't have rust but they might have bad paint (bleaching from the sun)
HTH
-pete
