What do you intend to do with the bus? Selecting the best bus for you depends a lot on how you are going to use it.
The bus you are looking at probably has among the worst ride of the busses that we commonly see here. The 5.9Cummins is a great motor. I have owned several including my current daily driver. Would I buy a full size bus with a 5.9? No. It is great in a short bus or pickup truck but is not well suited to a full size, highway bound, bus.
If you like the looks of the B600 and won't be driving it around the country then it may be a good fit. Or if your budget absolutely dictates a very inexpensive bus that may be your best option.
Again, knowing how you will use the bus will help with deciding which bus to buy.
Don't get in a hurry and settle for a bus that is not well suited to your application. I shopped the auctions daily for 18 months before I was successful biding on my bus. I bid on and lost more than a dozen before I was winning bidder at a good price. I found a Bluebird All American RE with the 8.3 Cummins and Allison MD-3060 and only 180k miles. Winning bid :$3450. They are out there if you look hard enough.
Good luck with your quest.