When we started looking for our second (current) van I drew up a checklist of the features we wanted. You will find it Here. It's most unlikely that it will match your requirements but you may find it useful to help you start a list of your own.
How do you intend to use it? Do you want something that you can park easily as you tootle around, or do you instead plan on using sites for a week or so at at time and use public, or other types of transport, to get about?
Do you want a coachbuilt or a panel van conversion? You could consider a 'classic' instead which would come in your budget range and hopefully not lose value so much as a non-classic one.
As well as the cost of the MH you need to put some money aside for insurance, MOT, servicing etc but most importantly repairs as you will surely have to do some with an older vehicle.
The only genuine answer is one you can afford that has the layout you can live with. Older vans in your price range may possibly have a portapotti rather than a cassette WC. You say you are ok with interiors, why not convert a van yourself, bed, cooker and loo will get you started then you can add to it as you find a need. That way you can spend a reasonable sum on a more modern base vehicle. however Coachbuilds in the 15-20 year old are usually well built and often low mileage. an early Hymer may be a bit more juicy than a modern van but it will be fitted to a good standard and I have seen some on ebay in your price range. Our last van was a 1981 and just this week have seen two 1973 vans on our campsite (1 over here for holidays from Germany) So they are about, will get you around but naturally like anything old needs care and attention on a more regular basis.