That's all good news concerning the leak so far. The real test should be when you can get on the road. It's not looking like a significant leak, so put that on the back burner until you can put in some road time.
As transmissions go, my guess is you're about in the same boat as the rest of us. I know nothing about the 540, but most of the rest of us have a 545. I'd have to assume that was the predecessor of our beloved 545s. It's touted as a workhorse and has been used for years. These last well with decent maintenance. It's got its drawbacks, such as basically freewheeling on downhill sloaps with little or no engine resistance.
Your transmission has apparently already been remanufactured according to the data plate. I couldn't read the dates. My guess is, if you had transmission problems and had to replace your transmission, you'd be pleased with the price of these transmissions, considering you could hook up a 545. They sell used between $300 and $500, or higher. It's not the best transmission out there, but it's common enough that it won't break the bank when you need a replacement. Of course you can get a certified replacement for big $$ if you want to go that route.
There are newer transmissions, but they have computers that need to talk to the engine computer. A lot of our mechanical buses have avoided the computers, so there's only a few improvement choices. When the choice is necessary we like the 643. It's just a little step up in transmissions and apparently hooks right up, without electronics. I'm sure there are more but I'm not the guy to talk to about those choices.