We have a 1983 Mainship 34 MKIII with the Perkins 165T. Newer motor for you is definitely a plus as long as it has been taken care of. We bought ours about 1.5 years ago and have been doing the usual maintenance that you do on any boat, especially one of that age. Mind you, I am doing updates and lots of unnecessary things just because I want to not because it's required. We live in Florida and the boat was on the west coast and we live on the east coast which made our maiden voyage over 220 miles, including 6 locks and 4 leisurely days on the water.
Buying the boat has NOT been a disappointing decision and the layout is done well and feels much larger on the inside than it is. We also found out, at least here in FL, that 34' is the sweet spot cost-wise as being under 36' is less expensive for haul outs, dockage, and insurance availability.
The fuel tanks on ours are original and fine, most things are easily accessible and we love the layout.
The previous owner had a recent survey and fixed all deficiencies noted (mostly trivial) and I found a local deisel mechanic/technician who went through the motor quite thoroughly.
The best upgrade we did was have a bow thruster intalled and now I look like a pro while backing into our slip. Having that installed took away any stress or anxiety out of docking even though I did it plenty of times without it before the install.