It is said that the #1 piston will oval out the cylinder and bend the rod. I purchased mine almost two years ago and it runs like a top, doesn't use oil gets great fuel economy. I've also collected some other owner's who don't understand these problems having more miles on their cars than mine.
There is a group in Canada which hammers this vehicle regularly. My theory is the cold weather affects the #1 piston as the coolant from the radiator enters the block there.
My recommendation... buy one that's been in the south and disconnect the EGR valve. Look for service records as usual.
I have seen several for sale in the Raleigh paper (purchased mine at Leith MB by the way)and the MB owners club magazine.
Search this site for 350 messages. I don't think you'll see many complaints.
1991 350 SDL