I've had three (or is it four? I can't remember but I think I had two that were nearly identical!) landy's in the past. My trouble is that I haven't had the mog for long enough to judge it properly. Having spent years reading about mogs and trying to figure out if I could afford one I've talked to a hell of a lot of owners though and the impression I get is that they are reasonable. They have a reputation for being complicated and troublesome, but that reputation seems to lie with the folk who don't actually have one!
My take on it is that as long as you get on top of the air system before you go they're usually no worse than anything else. Air leaks do seem quite common but I think that is down to seals standing still and hardening up? My clutch slave seems to have gone partly for that reason so for the time it takes, little bits like that are worth pulling apart for the sake of it and checking. A seal kit for mine was £20 so changing it is no big deal.
If the machine has been well serviced that is a big bonus. There are a few little places that I have seen cause problems for several owners - the portals for example hold a very small amount of oil so if they have leaked and not been checked it doesn't take long for them to run dry and get damaged. An ex MOD 1300L would most likely have things like this well covered unlike a smaller company owned machine that may have been neglected.