I'm a British national about to rent an apartment in Spain, initially for six months. What documentation do I need to obtain from the Spanish authorities?

From what I've read, it seems that an NIE would help reduce banking costs, but I'm not sure that I'm eligible for one. I have income from a property I own in the UK, which is let. I also intend to to do some IT and photography work from home. I can show that I have sufficient funds to support myself for six months and beyond.

I'd like to know what, if any documentation I need. Also, if there is any documentation that is not required, but would be good to have. It is quite likely that I'll want to stay in Spain well beyond 6 months.

Thanks,
Keith