Firstly check their fully laden weight. Under 3500 kg can be driven on a car licence ( B) . Over 3500kg is a truck licence (C or C1 ).
Secondly, check that the number of seatbelts match the number of bunks. Some 6 berths, have only 4 seatbelts because the extra 2 people put the weight over 3500kgs. This is verified by looking at the log book. S1 category listing , I think .
As for the base vehicle, Ford transit. The 2.2 will be front wheel drive, while the 2,4 is rear wheel drive. Check the tyres for the date of birth, tyre older than 5 years or so, can fail a crvt test. Also camper tyres are completely different to regular van tyres because they have a higher load rating index, more expensive.
There are a good few uk based Transit owner websites who allow public viewings of their Transit reviews, no harm in having a look.
You mentioned garage. I am presuming that you are talking about a commercial dealer. A habitation report containing a certified gas report is the minimum and moisture ingress. In other words, the gas is safe, and the house doesn't leak. The usual engine service and timing belt change or timing chain checked along with a new doe test.
The big issue with 6 berth campers is the little people. My kids break everything including me. Open each door, window, roof vent, lock, latch, catch etc multiple times, as the glue may break.
Likewise turn on each switch, knob and appliances multiple times and max out the gas and electric hook up. The more stuff on, the more things that can go wrong.
Lastly for this time of the year, a 3 month warranty is useless because the wagon will not be probably out every week.
Just a few of my thoughts,
Malcolm