I disagree with Scotty on buying used Cummins trucks...sure you have to be careful but if the seller knows something about the truck and it is clean it will be fine. The biggest thing on the Cummins truck is the beaters and the pumps on the soft serve machine. If they are in good shape and the machine gets cold it should be easily serviced. Not getting cold...could simply be it is out of Freon...it happens. Belts are the other wear items and David Cummins stocks them or most are readily available automotive belts. If someone is selling a used Cummins truck and has spare O-rings and belts then they more than likely know what they are doing.
I bought a used Cummins truck and just sold it this past year...it was in great shape when I bought it and I feel when I sold it in was in better than good shape. Sure people can neglect them but they are really robust....and not that big a deal to service...my local HVAC commercial guy took care of me if I had a cooling problem....which I did once in 3 years.
I had more problems with mix than I did with the truck when it gets down to it. If the inside is neglected....yes stay away....then likely the soft serve machine was also.