Is using a trailer not a option? You could buy a cheap normal trailer or go to a box one. Great if you want to pick something up, use for camping or whatever and without the extra cost of running another motor.
If it was me I'd be looking for a transit. Older one with banana engine. Slow, easy to maintain, plenty of parts. No injector or turbo problems like on newer ones. You could always turn it in to a day van or camper if you wanted. Buy your transit, you could pick one up for a few hundred quid if you look around. Then buy a caravan that has damp issues or just a carp shell that leaks, rip everything out of caravan and fit what you want in transit. You could make some money back from bits of caravan you don't use like Windows, chassis, toilet, water heater, cooker, fridge etc. Just look what the bits I've mentioned go for! You may even make enough back to buy you another retro bike.
Leigh