This is the aproach I took.
1-Purchase the unit that seems in decent and more or less usable condition at a price that felt fair.
2-Clean and familarise with systems, make list of needed repairs and wants
3-Go camping, use all tanks, look at what needs to get updated or modified, go back to list and rearange accordingly.
I made a point of going camping often, thats what I bought the unit for, there more I went the more the priority changed on what repairs, updates and wants where/are on the list. The water pump started leaking so it moved way up on the list. 110v for the fridge stopped working, it was on the list for replacement some time down the road do to its age but the heating element was cheap in comparison, I'm still using the same fridge 2 years later
. Get rid of carpet and find the need for floor repair. And so it goes on. My list still has items on there from its conception that have not been adressed. I try to keep to a reasonable budget on needed trailer repair and upgrades for the year, sometimes I make it and other times I go over.
It depends on the people and their means.