As far as Ward bus bodies are concerned, it sort of depends upon when they were built that determines whether they were good or not. The last few years before Navistar purchased a part of the business and until Navistar owned the whole business the quality was all over the map. Surprisingly, water leaks were never a big problem for them considering how poor the quality control was.
Ward cut a lot of corners in quality of materials and products used in order to stay competitive. But their basic design didn't change that much over the years so most of the Ward buses held up about as well as any other bus.
As others have mentioned, the GM 366 truck engine is a great engine for what it is.
It has virtually nothing in common with the big block engines used in GM cars and light trucks. It may look similar on the outside but inside it was designed and built for a lot more abuse than the engines used in cars and light trucks.
Because it is a GM big block getting parts and pieces to keep it going are relatively easy to find and not that expensive.
The expensive part comes in with the stops at the gas station.
Even though the bus is relatively small it will have a pretty big appetite for gasoline. Figure on 6 MPG tops with speeds less than 50 MPH. Increase speed to 60 MPH and you will be down below 5 MPG. And those guestimates are with everything working at tip top condition with fresh gas that isn't an ethanol blend with water in it. Realistic fuel mileage would be in the 4 MPG range. Or about $0.75 per mile just for fuel.
The same size bus with a T444(E) or 6BT/ISB will get 8-12 MPG.