Hello as well Grant and welcome aboard;
Despite the fact that you can weld (I wish I could), I wouldn't go near to the grandest rust buckets on the open road because if you cure all the rust and make it look nice, then as sure as eggs is eggs, it will get pinched!!
If you can I would go for a van that has a fully galvanised body which means the Renault Master (+ Vauxhall & Nissan equivalents) from 2004 onwards or the Sevel built vans that went fully galvanised from 2007 (Peugeot Boxer, Citroen Relay, Fiat Ducato).
Most of these are front wheel drive which results in a lower load platform.
Arguably the best ones to go for are the Sevel vans as they have the widest and squarest bodies which make them the easiest to convert......... But having said that both Edina (Chris) and I went for the Renault Master and we both love them to bits.
Phil