Hi Pete and Mrs Pete and welcome to Eye on Spain!
You wanted a 'kind' reply, so here I am with the first one, though I'm certain you will receive more 'pointed' replys, which may be more gloomy.
Your question is quite broad really, and I don't have the knowledge to answer the 'what do I need' one in full about long term rental. For sure, if you look at the long term rentals on here, or any other Spanish information site, there are lots and lots to choose from, depending on what you are looking for, a simple 1 bed apartment in some area's might be 300e per month, upwards. Where you should look is a hard one to decide isn't it. Do you want to be in a buzzy place, with English speaking people, or away from it all somewhere very rural. I'd probably advise not isolating yourself too much, especially if you are used to socialising in the UK, you and Mrs Pete might get on each others nerves otherwise!
The main thing, as everyone will tell you, is to have enough finances to support yourself comfortably for probably longer than your planned 11 months. Quite simply, there are no jobs for the Spanish, let alone anyone hoping to earn a crust putting their skills to use as an expat, and no friendly Job Centre to slip you a bit of Jobseekers Allowance or Housing Benefit.
The plus side of a long stay in Spain is that it will give you chance to see if you might want to make a more permanent move and you will be able to travel around a bit once you get settled in your rental place.. The weather obviously, but don't for one minute think that it's hot all year round. It can get bone chillingly cold just the same as the UK. Cost of living, in MY experience is less than the UK, as long as you live like you would normal life, and not as if you are on a long term holiday !
I think you will have to resign yourself to making a few trips over before you 'choose', it could be a costly mistake otherwise. I'm sure others will have more indepth advice for you. GOOD LUCK, and let us know how you get on.
Rosie/RiojaRosie.