REME 245, from my limited research it seems any sort of Welfare vehicles. Be they NAAFI, Red Cross, YMCA etc are extremely rare anywhere in the world. In Australia there is a Jeep that was used by Red Shield/Salvation Army and there's a CMP truck that has Australian Comforts Fund markings still visible on the door. But that is nearly it, globally. As for the suitable chassis/cab that is the direction we will likely go. As I'm in the US I was looking at either having to import something or "make do". But I've recently identified 3 distinct types of vehicles that were exported from the US to England in 1941 of which at least 15-20 saw service with The Salvation Army and others appear with Church Army, YMCA and other entities. I don't know how many total were exported. But they are distinctly US vehicles complete with "CAUTION LEFT HAND DRIVE" stenciled on the rear. Building my own based on one of these is probably my best option.
As for the cap device. Good call. That's a current one I ordered through Salvationist Supplies. I have some older ones, but I need to get an older Soldiers one (blue) and not Officer's (gold) most period ones I come across are Officer's type. The cap is also one of the low quality reproductions and I'm not thrilled with it. The Battle Dress is original and patches on it are reproduction.
Matthew