I see the signs of rust too. I'd check and see how rusty the underside is before anything else. If that's a go, then I'd note the following:
Tires don't look "almost new". They do have acceptable tread and if you don't drive it much, you may never wear them out.
Rusty mentioned an AT545 transmission. I don't think so based on (1) DT466 engine (rarely if ever mated to an AT545), (2) shifter has D, 4, 2, 1 (indicating at least 5 forward speeds), and (3) being a 2001 model, the AT545 was quite rare for new buses (if available at all). Whatever it is, it will probably be an overdrive transmission.
Battery looks to be an 8D type. While these are adequate for starting and operating a bus, they *ARE* rather pricey and very heavy. It can be replaced with a pair or trio of group 31 batteries when it's time for replacement.