Mobile home may be the high risk option as if you buy then later you may have a problem in selling on
Also permits need a lot of checking
A motor home may be better and they are certainly very popular in parts of Spain....in parts of Almeria you can take them right down beach side
I do not know you budget nor infact the location you like best but when you are over in Feb you may as well have a look at properties as you may just find something suitable
I think 2008 will be a good time to buy re sales in Spain and you should be able to pick up a good location at a sensible price