
 A few points to consider.
Vinyl wrap comes in various grades, the fully conformable vinyls are about £20 per meter off a 1.5 meter wide roll _This is for standard colours
Printable conformable vinyl is designed to have a protective laminate applied after printing, It too must be a high end conformable vinyl
A set of Vinyl & Laminate, design costs, printing cost and lamination will bring the cost per linear meter up towards £40
Both sides, back and front will use a minimum of 14 meters another 5 meters for the roof of a tin top - do the maths!! The £800 job is only possible if the Guy uses cheaper vinyl, this will lift in time 
If the laminate is applied  to the print too soon after printing the solvents will have an adverse effect on the adhesive, which will lead to premature
failure of the print and will make removing the vinyl from the van extremely labour intensive. If it is factory paint there is a good chance of a successful clean up, but on resprayed areas there is a strong probability of some paint loss.
Another point is that when vinyl is removed, it is not unknown to have knife marks in the paintwork and the rust is already at work, typically this occurs when joints in the vinyl are hidden in body folds 
 The joints on our vans rust , primarily because of the cracking of the sealant away from the metal, this was applied prior to painting , so when it separates from the metal it leaves an opportunity for the tin worm to get busy, this then spreads along the joint and around onto the face of the joint. It was used because that was industry standard at the time and would give a ten year service life, meanwhile the inside of the joint is exposed to condensation from sleeping inside , remember when your windows have condensation on them , so has the inside of the panels! and in the case of the joints exposed behind the rear wheels, road water, salt etc usually do the honours
Modern vans eg Merc Sprinter Transit etc paint the vans first and then use a rubber jointing material, which is left black, this is more flexible, but the fact that it is going over a fully painted surface means that even if the seal fails, the metal is protected
All our vans are minimum 20 years old & all have some rust , even if it hasnt popped out yet, a vinyl wrap will accelerate this, as it further prevents water from drying out, even if all fittings, windows etc were removed and the wrap was applied under everything the condensation etc work away at your joints. 
and I know all this because I work in this industry, Vinyl wrap is great for advertising, it can look cool , but it is not a rust inhibiting product!
Lastly if you need to sell your bus, who is going to buy it, not knowing what is underneath?
Fighting rust is a rearguard action, and if money is tight learn to do some remedial / preventative work yourself while you save up for a proper job. Keep drain holes clear, waxoil or dinatrol from the inside using a heat gun or hairdryer to dry joints first. Leave windows and doors open when possible during/after sleeping overnight to allow air through the van, pull you step rubbers off periodically and give the steps a lash of waxoil, dont garage it when its wet and when you do get to the bodyshop the bill wont be so big 
