Evening Ollie, the best advise I can give you is the same I would give to any body else and that is to buy the best you can. the better the van to begin with the less you will need to do to it to keep it on the road. Even with a good vehicle there will be plenty of jobs to keep you occupied as most old cars tend to be somewhat labour intensive. I can see that you are mad keen but try not to start with a complete restoration project unless you have the time, facilities and cash to fund a long term commitment, a project of this size could overwhelm you and you don't want to be half way through and find your resources have dried up. Prices vary immensely from a couple of hundred for what would be worth more as a pile of spares to £4K + but remember what may seem to be the cheaper option will consume the ££££'s very quickly and to finish it you will probably end up spending more than the cost of a decent one. There are still bargains out there, there is more to buying Commers than checking eBay. Check out the Autotrader and classic car magazines also the local papers, you may get lucky. It could be a long wait to find the right one but don't dive in and grab the first one you see (unless it's a good 'un) but take your time, after all it will be a while before you can drive it so start saving and keep Commer hunting.
'Harvey the RV'