Hi Peter, welcome!

Can't advise well on mechanicals, but having just bought mine, try using the links on this site to get to others and browse as many "sales" sites as possible, to give you a good idea of types and layouts.
munichkampers.co.uk and links from just kampers are good for examples of vans. I should also check out the wiki (top of page, little hammer symbol) for advice for new owners/prospective buyers. Also browse this site for buying advice (lots on here from previous newbies), and heed the bodywork in good condition comments.
Where to buy? Well check this site for sales and to obtain relative prices, listen to the moderators advice and other regular contributers (you will soon recognise them from old threads). Anything from Mr Simon Baxter carries great weight! If you can find someone to show you a vehicle and point out potential problem areas before you buy would be excellent. Caution with ebay, and good hunting. Hope this has been of some use.
(any moderators please feel free to correct if this is misplaced advice

)
Myk.