First, I don't let in Spain - at all, ever. I think the idea that there is money in rental property spills over from the UK where it is pretty straight forward to run a reasonable business, so I just wanted to say there is a middle, more common scenario, in that people rent out their place in Spain just to help meet costs. Some overstretched themselves in buying in the first place, and some did not intend to keep property, they simply wanted to flip it and make a quick profit. I would say that cheap kitting out an apartment is not going to fly. The competition is just too great, with some really good quality property around. Second, if you want to be sure you are legal, there is a lot to do, and I know people who used to rent out, and now say they will not because of changes to the law, agitation by hotels etc. Others on here will know much more than me about that. The most successful holiday rentals around me are where people run them personally, and promote themselves with their own websites. I think a poster has already said that the peak season is only 8 weeks. So there is not a lot of money in it as far as I can see.
HOWEVER, if you have children, you might like to buy a nice really cheap apartment and sit on it for them. In a couple of decades it could well be worth something - maybe!!