Personally I think your still better off with an ignition switch, but just a universal one would do.
You would still have some security and its easiest to wire up.
You rightly could just use two switches but the ignition switch would have to be to be a high current rated one depending on what circuits you still have (being an offroader only).
The push button one would still have to be a reasonable rated one.
You can get a plate with the 2 switches on already on fleebay for about a tenner.
Again a key one is about the same, but most on fleebay don't say there current rating.
A photo of the wires connected to yours may be helpful as I'm not sure (conflicting info) what the Spanish wiring colours are.
A pic of the fuseboard would be useful as well if possible.
Something like this
http://www.12voltplanet.co.uk/4-positio ... witch.html