These bikes are loved by long term owners and hated by just about everyone else. It's the only Iron Curtain hack that has passable looks and can actually be fun to ride.
You're not going to get very much for fifty quid - an early seventies TS is the best bet. These have useless front brakes - a common replacement is the front brake off an old Honda 250 twin, a much superior TLS drum - and main bearings that can last for as little as 5000 miles, or may give no trouble whatsoever. A re-bore is needed somewhere between thirty and fifty thousand depending on neglect and the level of thrashing.
Where the TS does score is in frame and suspension, which for the time was actually way ahead of the Jap commuters (or even so-called sportsters).
The TS will run up to about 75mph on a flat road, but, more importantly, can be cruised at a useful 70mph for as long as the engine will stay together. Maintenance is all straightforward of the large hammer and cigarette paper variety. Some bikes are really well looked after and worth buying despite high mileages - there's a sort of informal competition amongst MZ owners to get a hundred grand on the clock.
Electrics are a little dodgy with the occasional burnt out bike, but they have usually been sorted by this mileage. The gearbox gives problems from 40000 miles on due to selector wear and gear teeth fatigue.
Generally speaking, the newer the bike the better it is, although many MZ owners are not too keen on the looks of the latest model. The older bikes do look quite butch and purposeful. It'll take a test ride to find out if the idiosyncrasies of the MZ is to your liking or not. As the 125 was the second largest selling bike in '86, many people seem willing to overcome that particular problem, I wouldn't buy one, but many people do.