What are the benefits of being Resident in Spain?

I would suggest you contact a Spanish financial specialist as you may get a lot of individual opinions which may be helpful but tax laws and residential laws change so you need to get the best up-to-date advice possible which if not up-to-date could be costly

Provided you sell your house in France before taking up residency in Spain there would be no taxes in Spain. As soon as you have Spanish residency you become liable to income and other taxes in that year.
It's exactly what we did coming from UK. Although we had our house in Spain, we were still resident in UK. Sold the house in UK in December, took out Spanish residency in January so nothing to declare.
As scubydoo says, check with a solicitor or financial advisor in case selling in one year would need to be declared but, if you're selling in France at a loss, can't see if any CGT would apply.
Spanish tax year is Jan to Dec.