We started out much as you are - coming from Canada and renting for 3 'winter' months in self-catering accommodations (the British tourist way of saying renting a cottage ) We stayed in Santo Estevão , just outside of Tavira. Loved the area and nearly ended up buying there, but fortunately the deal fell through.
I say fortunately because we were 'settling', thinking it was the farthest south we could get, that we were going to have to be content with needing some heat in the winter, and with prices close to the sea being outside our budget. Reminded us of Florida weather-wise but not culture-wise.
Long story short, we ended up in Madeira where we are happier than the proverbial pig. However, each to their own - just to say that I think the way you are going about this makes absolute sense - it is the plan that worked well for us.
I tried to look up the place we stayed on http://www.homeaway.co.uk/ with no luck, but there are lots of possibilities. Most people with short term rentals are happy to do long term, especially in the 'winter months'. Only need to email and ask.
Suggest you look for a 'cottage/bungalow' if there are just the two of you, because the bigger houses need more heat to be comfortable. Most houses are not really built for cooler times and usually do not have central heat (nor do they really need it).
And not to help you steal the thread snowbuddy, but we are all among Canadians here eh? Lots of our friends have bought in Mexico, because that is a natural for Canadians. They love their places, but bought thinking they would do 6 months there and 6 months in Canada (as we thought when we started out) and now find the summers are too hot to be bearable if they want to stay longer. They are mostly in Puerto Vallarta, and we go down to visit them every February - when the weather is lovely. Don't know how that compares to Playa del Carmen. We seriously considered Mexico, but even with being hyper careful, every time when we went I got food poisoning, so that finished that option for us. Our friends that live there all routinely go to get checked by the labs every 6 months - part of normal health routine for them. Cost of flights etc about the same, cost of living maybe a cheaper in Mexico. Again to each their own - our friends are very happy with their choices.