Unlike some here who don't think much about the AT545 transmission, if you understand it you should have no problems with it.
I have seen those work in bus just like yours well over 250,000 with nothing more than regular fluid and filter changes.
Like any automatic transmission, DO NOT get it hot. Heat will kill it faster than anything else.
When those Blue Birds first saw service we had a lot of problems with the transmissions overheating. Come to find out, Blue Birds built for service back east don't need the extra transmission cooling that is required of buses that have to go up and down some serious hills.
I am sure that as old as your bus is that problem was addressed a looooooooooong time ago and shouldn't be a problem for you in the future. Just watch the trans temp gauge.
In my experience the biggest drawback is the AT545 doesn't hold back on a hill very well. Unless you really downshift into the lower gears you get virtually no engine braking. Without engine braking you are going to use up your service brakes pretty quickly.
If the choice of buses was between two buses of similar quality and price and one had the AT545 and the other had the MT643 I would go with the bus with the MT643. But I would not rule out a bus with an AT545 just because is had an AT545.
Depending on your rear gearing the four gears in your AT545 will be more than adequate for you unless you intend to go well off of the end of the pavement with it or go up high with the mountain goats.
By the way, your bus has air brakes with a very strong spring brake parking brake. As long as your brakes are adjusted properly a spring brake will hold a bus parked a lot better than any park pin could.
Good luck and happy trails!