Hi KWM2. I see nobody has responded to your post so I will. I've been looking for a bus to convert for several years. I recently found one locally, New Jersey, that I bought and am just beginning to strip for conversion.
The bus in the ad you posted would be a good chassis. A rust free bus is critical unless you are good at, and like, fixing rusty sheet metal. It's a flat floor too which removes the wheel wells from the equation when building it out. Being a flat nose provides more useable space inside in relation to overall length when compared to a bus with a 'dog house'
I was looking at a couple buses with Cat engines. Go to Youtube and search for 'adept ape' and learn about Cat engines. The Allison 2000 transmission is a 5 speed automatic, overdrive tranny which are ubiquitous, a good tranny (caveat, if properly maintained). 155K miles is low which is good. The overdrive gear will provide lower rpm at speed compared to a tranny, like the AT545, which is a 1:1 top gear.
One thing about the later engines that I studied a lot was the HUEI injection system. A number of engines used this system and proper engine oil maintenance intervals is critical.
The bus in the ad is a front engine (FE) so you can expect to get more noise than with a rear engine (RE) or dog house (conventional front engine). Personally I wouldn't be averse to a bus like that though maintenance on the engine might be tighter given that configuration when compared to a tilt nose (conventional bus).
When looking at a bus check the DOT age markings on the tires. It's very expensive to replace 6 bus tires if they're too old for safety.
Yes the bus in the ad is priced high. On the other hand if it's in good condition with good rubber, no rust, well maintained it could end up being less expensive than a clapped out rust bucket at half the money. If you're in the Phoenix area you might want to take some cash and see what you can do on getting the price down. If you're across the country it becomes more difficult, though there are folks that have found drivers that will deliver a bus across the country. My bus was owned by a bus company about 50 miles from where I needed it to be so I got the owner to have one of his drivers deliver the bus. I paid the driver $100 which was nothing compared to having it towed or driving without insurance which I wouldn't do under any circumstances.