Just checking for water damage would be your only real thing to be certain to check for.
As long as all the appliances fire on all the modes and all the switches and dohickeys work you're pretty much just looking again for possible water entry points that have occurred over the years.
As far as a dome, I doubt the dealer has the dome sitting around that would with a 2009 151 any longer. The domes for those units have a smaller attachment point than the current domes and also back then they used smaller domes because of how small the 15x models were.
Today a new dome from the dealer would be a 13' model with a large keder.
You might have the best luck finding a true fit dome directly from Forest River. It would for sure cost less than what you dealer is saying and they might in fact still have the old Rdomes in their storage!