During the winter, but I wouldn't buy my first van from a dealer. Look out for (locally if possible), an elderly couple who are giving up caravanning, either for health reasons, or age.
Two reasons why I recommend this, one because it will fairly certainly have been looked after, and two, you might well get the kit to go with it, an awning perhaps, or chairs and picnic table. You might just fall in lucky and get the whole kit and kaboodle, and believe me, that would save you a whole bundle of money! From a dealer you'd probably get just the bare van.Yes, you'd get a bit of warranty, but I'd personally rather have the goodies!
Good luck in your search, whatever you decide, and keep us informed, please.