I think it would be premature to choose a particular make bus when your budget is of primary concern.
It would seem to me the prudent action would be to seek out ALL coaches in your price range & choose the best from what you find. Sometimes, the best money spent on a bus is the return trip home after looking at one. . . .
There are some really nice units out there to be had for a song - especially if you know what to look for.
Gary Labombard made a CD that documented all the places on his bus that had rust & how he dealt with the corrosion. http://community.webshots.com/user/converter101
The GM buses are lighter than the other brands (Bluebird being one of the heaviest) & less weight cost less to move down the road.
A friend has PD4501-738 & gets 10+ mpg with a 8V71 & 4 speed manual. It weighed 25,000 after he removed the seats. . . .
He is retired & doesn't get in a hurry going down the roads. . .
Believe what you want, but the facts are that the GM PD4501 set the standard for all buses that followed:
first with standard A/C
first with standard washroom
first with HUGE bays
first 40 footer
power steering was also standard
supported by 10 tires & 12 air bags = very smooth ride
only GM PD series bus with a T-drive
under 11' tall
Most distinctive bus ever built.
However, the dropped aisle made for a terrible conversion choice.
Greyhound used them for 20 years before selling them off beginning in the early mid 70's. You can bet they were in need of maintenance when they were sold - & also a safe bet most didn't get the proper service after leaving greyhound . . . .