Really depends on when you're looking at coming Charly. During the months of summer ie July/August some of the beaches have restrictions placed upon them, or are strictly zoned as to sunbathing, kitesurfing, windsurfing...that's not to say there's no kitesurfing beaches just that you'll have to search a little harder for them. During the months of summer the police take a noticeably dimmer view of people sleeping in vans on the beaches...this is for several reasons I imagine, litter, fires, broken glass and the local councils wanting the campsites to be full. Again doesn't mean you can't do it in the summer...just that you'll have to be a little more discreet.
Outside the summer it's much easier, most beaches are open for kitesurfing (at least along the Costa de la luz and the Costa del sol...can't really speak for the rest of Spain) and kitesurfers are pretty much left to regulate themselves. Sleeping in a van is also much easier especially if you move around a little.
I'm based in Valencia (east of spain). Don´t go to barcelona (whole Cataluña indeed) because they have very strong restrictions during summer time.
In Valencia (region Comunidad Valenciana) you have restrictions during summer but in general you can find a spot every 40-50 km maximum with an allowed to kitesurf regulated zone. You MUST be in one of these zones in order to kitesurf freely. Surfable winds are those coming from any E direction, (usually SE in summer is the local wind), between 10-22 knots tipically in summer (would say 2-3 days per week depending on the week, check windguru).
Just for your info, these zones are opened to everyone to do kitesurf, however there is usually a local club who take care of the zones. You don´t need to be a member in order to surf
, but it is allways advisable to present yourself and ask them for special conditions of the zone (dangers, climate, winds, be nice and follow the rules in general...)
Among those zones, there are: Saler, Sidi Saler, Barraquetes, Tavernes, Gandia, Oliva (Piles), Calpe, Santa Pola. I guess you can find them on splocs. Going to the south (Murcia) there are nice spots in Mar Menor.
Regarding Tarifa, take into account it is a very different kind of spot. Stronger winds (20-35knots even more some days) which many days come from the E so they are offshore. It is a very more skill demanding region from my point of view. Crowded of kites/windsurfers in summer.
Last but not least, there is www.spainkiters.com
where you can find different subgroups for each of the zones of spain. You will find more detailed info asking there. We are not a great english spaking country, but I am sure someone will read you and reply with info.