I am no expert in long term rentals but I suspect that they are available locally, so you need to go over and start searching
A lot of the rentals on the internet are short/ medium term
I appreciate your concerns but perhaps you should rent short / medium term say for 3/9 months [ just come back for UK summer] in a furnished property and then get some experience of living in Spain and you may find buying with the proper advice is Ok for you
Renting long term and taking your furniture over also involves all the regulations and factors you have to consider for being resident in Spain and of course you will have to make sure that you get a lease of a secure length
I understand that leases of more than 11 months have problems for landlords so you may be restricted to this length
To buy a decent property prices start at circa 104.000 € for 2 beds