I would start with how much money do you want to spend? Will you do most of the repair work yourself? Will you use the MH for extended trips or just local camp outs? Older MHs will require a lot of work if the PO has not up graded or maintained the MH. If you have $40k or more to spend look for a 2006 or newer gas MH Ford or Chevy. These will have the best drive trains(eng & trans). There were no chassis improvement after that year. Delaminations seem to be less of a problem with the newer MH's also. If buying older it is all about the condition as others have said.
