cruise22,
Personally with regards to a Ford E350 and E450 based motor home......
With limited exception, I would not consider a motor home older than a 2003 to avoid the engine spark plug head-thread issues.
A Ford E350 and E450 prior to 1998 will lack much, but will still be fairly reliable as long as it is in good condition.
The transmission has gotten better and better along the way. Any transmission regardless of model year have a good reputation, but the older ones lack an extra gear (or 2 pending how old) resulting in lesser gearing behavior, and are less fuel efficient.
There are more recent thresholds. 2009 was the first year with a more capable chassis along with the current day dash board with glove box and better 12V outlet positioning. If the rig you are considering is on the heavy side, a 2009 will be a wiser choice. If the rig is lighter-weighted like ours, the excessive capability will just yield a rougher ride.