If the ambulance has twin rear wheels – i.e. a pair on each side for back axle then it will be over a 3.5 tonne rating. However, you can get most commercial vehciles downgraded through the DVLA to a lower weight class. You’d need to ensure the weight you reduce it to is adequate for you and your conversion needs as if you get pulled into a weigh bridge by VOSA and are over its a big fine.
The frontline ambulances are normally 4.5 tonne Mercs etc. They can come with a hell of a lot of redundant wiring and gas plumbing and air suspension etc.
Plus points would be maintenace regime and well insulated.
Some of the patient transport mini-bus type ambulance that are just a panel van rather than a UVG plastic body look quite good.
Have a look at Merthyr Motor Vehicle Auctions for loads of choice of ambulances.
My top choice though would be an ex-Police van – search on van trader for Police Mercedes. They can come with insulation, windows, lighting and night heaters. Much lower mileage than ambulances and better servicing regime.
vantrader police Merc