The brake problem is a bit of a self-inflicted injury caused by the parking brake being left on for long periods. Moravan never intended it to be used like that and the Flight Manual makes it quite clear that, for long term parking, the wheels should be chocked and the 'park' brake should be released. The recurrent problems that OAT have with their aircrafts' brakes do not seem to happen elsewhere.
Ingress of rainwater is a problem. It gets in around the locating spiggots of the sliding canopy and there is a fix. However, provided that the canopy cover is properly fitted, it shouldn't be too bad.
The ban on flick manoeuvres applies only to UK registered aircraft. In fact, the aircraft flicks very well and is totally predictable. The elevator 'overbalance' is nothing special, so long as you are prepared for it, and spin recovery is quite straightforward.
The biggest problem is the very limited life on the main spar. Both Oxford's aircraft have been back to the manufacturer for rectification work at significant cost. It will be worth checking the remaining life on the aircraft at Gloucester, if it's G-OZLN then there shouldn't be too much of a problem. As I recall, the first two owners were not exactly enthusiastic aerobatic pilots and it won't have built up too many aerobatic hours.
Having flown 5 different airframes, including the prototype, I'd rate it a little above the Bulldog, of course it does have full inverted capability although the negative 'g' performance is a bit 'iffy'. A full inverted loop is possible but you've got to get everything just right. Inverted spin and recovery is a breeze.
If you want any further info, e-mail me.