I am not a GM enthusiast. I have an MC5a, but do not consider myself to be necessarily loyal to MCI either, so I will only address generic concerns. If you have a bus that is low on power, has a manual transmission, and may be difficult to get parts for, you live with, and work with those attributes. However, if you are looking to buy a bus, you have the oportunity to get as many features as you want upfront. Unless you really desire the joy of a manual transmission that is ill suited to RVing, look for one with an Allison. Unless you want to be the absolutely most anemic vehicle on the road, look for one with more power. Just so you understand, an 8V71 is underpowered unless it has a turbo. The one joy you will have when slowly chugging up a grade with an 8 is the knowledge that at least you do not have a 6!
The purchase price is really important, but is unlikely to be your biggest expense over the first couple of years. It is far cheaper to get what you want to begin with, even if it costs a bit extra, than to try to upgrade later on. For example, you may have to pay an extra thousand or two for an Allison and bigger engine when buying the bus, but upgrading will cost a prohibitive amount more.