Hi, and welcome. You have to also factor in your utility bills. Electricity, even for the frugal, will be around a hundred euro a month. Yes, 100. Water and rubbish collection about 30 euro a month or more. So that's at least 130 out of your 750 left. It wouldn't really leave you that much for spending too much time in a bar.
We have discuss this in previous posts and the consensus was that a thousand euro a month would give you a fairly good lifestyle but that was without paying rent. Remember, if you are only staying for up to 3 months at a time (and you could only do that twice year) you would have to save for flights going home as well. You also have to take into account your health situation as you would only be covered for emergency treatment in Spain.
I would be one of the last to put anyone off coming to live in Spain as we absolutely love it here and the life. OK, there are two of us and we get through about 800 euro a month which covers food, a bit of travel, eating out on a fairly regular basis. But we have no rent, local taxes and insurances are separate to that as is buying anything large we may need. I do feel you would be scraping the barrel a bit.
I know it seems a long time off until you're 66 but maybe just visiting your sister occasionally until the state pension kicks in which would go a long way to allowing you to live that bit more comfortably.