Ambulances... from first hand experience they’re worked very hard, obviously. Worth keeping an eye on Merthyr auctions, they get all types of interesting vehicles going through, not just ex nhs ambulances. Check out suppliers of ex mod vehicles too, been some interesting ex nato stock about in recent year.
Also worth checking autotrader for specialist body types as odd things popo up occasionally.
2 ones I saw after I bought mine was a sprinter surveillance vehicle with a whisper generator (5k on its own) also saw a 7.5 ton rigid body converted by the police and already registered as a Motorhome by them... As it had some bunks in it and a shower.
I looked at several ambulances and fridge vans before I bought my crafter but was put off all of them... why would anyone buy a van with 400000 miles on it unless it was extremely cheap?...surely everything from the bearings up will be approaching end of life?...also with ambulances there is serious amount of unnecessary weight in redundant wiring and hardware. For me it was the smell of the ambulance too that put me off!!