Hellop Sooz
Best advice is to look at as many campervans as possible to try and find out the type (conversion or coachbuilt) and layout that you like. Remember, however, that any camper has to be a compromise between what the designer supplies and what the camper needs.
Try out all facilities. Making the beds can be difficult for some people in some vans, while in others it may be impossible to use the wardrobe or the cooker when the beds are made. Try to find a layout that you think will cause you least inconvenience.
Regarding sites, in the UK it is illegal to camp anywhere you do not have permission.
Roadside verges, laybyes, car parks are all owned by some local authority, none of whom give permission for them to be used as campsites. Doing so tends to upset other people who regard themselves as "normal" and hence casravan/motorhome users tend to be classed along with the travellers and hippies.
There are plenty of "green-field" sites (such as those at folk festivals) at about a fiver a night, which is all that is needed for a self-contained unit such as a camper. The high-spec holiday sites are usually much more expensive and have facilities that you will probably never use.
Buy a few copies of Motorcaravan and Motorhome Monthly for some excellent articles on vans, maintenance, insurance, sites and most other things that a beginner needs.
Best of luck
Colyn.