As far as I can tell lots of people rent property in Spain for longer than 6 months and don't bother registering for an NIE. Owners are only too happy to rent out 1 to 4 bedroom apartments for 3 to 400 euros a month and be paid in cash. If you want to be legal then just do a visa run every 3 months and return the next day. And finding an apartment is easy. Just book into a hotel and then ask in the local bars. Renting a car used to be cheap in Spain but no more unfortunately. So that's the real problem - no NIE, no car purchase. Good luck!