My take on Mil Surp generators right now:
If you're planning on using a set regularly (at least once a month) the MEP-80x series have a lot of creature features (and safety items) that the older sets lack. They're being currently sold off. Many are rough. They don't sit unused well, and the water-in-the-stack problem seems to be a real design flaw.
There's no difference in power quality between any of the four pole synchronous generators (MEPs put out very clean power) so that's of no concern.
People like me who get around to running the set four times a year prefer an older, simpler set with less to go wrong/be maintained: MEP-002a or MEP-003a, MEP-016/701a, etc. Particularly, the lack of liquid coolant, belts, required alternator, etc are a big feature in this use case. I can run the gen set on battery for many hours with no working charging system because holding the cutoff solenoid open and the fuel pumps are the only real draw in the system. My wall-wart battery tender plugged into the convenience outlet will keep batteries topped up while running if the internal 12v PM generator or its regulator dies.
My MEP-002a is rated at 5KW (continuous, 8000 feet elevation, 100+ F degree day), but will reliably put out 7KW continuous, and even more intermittently. It will start very big motors for a generator of its rated size. It will, for instance, run both heat pumps in AC mode, and can cope with one in heat mode, even though it's technically not rated for peak output that high. I like that it has half the fuel consumption of its 10KW big brother as I have had to hump fuel up in 5 gallon totes when the road was too bad for a wheeled vehicle ('snowpocalypse' of 2009).
Mil Surp standby power above 10KW gets complicated - the MEP-004/005 sets are even more complex/bigger than the MEP-80x sets are (lots of expensive things to break) and have a high idle fuel burn. I would not use one unless I really needed more than 10KW continuous output and there was no way to subdivide the load onto smaller sets.