Barbara,
we do alot of static caravans and sometimes find the fuse box will not take the power of large machine, many are downrated for safety reasons, so would be good to have a plan b just in case!
As for pricing we find most prefer the cost for the whole unit, if you price for area you'll find most statics don't have too much carpet but it will still take a while to do as it is fiddly ( a smaller stair wand is ideal for the small bedrooms and bathrooms!) - same goes for the seating,
regards
steve