I personally think you would be wise to keep your house if you can.
My advice would be to do your figures carefully before you go down the remortgaging road. Make sure the rent you receive will cover the mortgage and factor in any worse case scenarios. Any shortfalls you will have to pay and it may be sometime before you are both earning enough to cover this.
Choose your agent well, we didn't and lost 6 months rental when he kept the monies he had received in advance.
It is also very rare to have one renter move out and another move in so there will be a month or so between rental periods. Can you afford to pay the mortgage?
In Spain you won't be offered a rental for more than 11 months as over this period of time you have security of tenure.
As you're not sure on if and where you want to settle down it may be more reaslistic to go for shorter rentals.
As with buying there are plenty of rentals around so you should be able to get a good deal.
When you do decide you want to buy then another thing you should factor in to the buying expenses is how much it is going to cost to run it yearly.
The biggest charge can be if you are on an urbanisation as these charges can be quite high depending on the facilities available.
If you have a community pool which is open all year round then you are going to pay more than one which is only open for 4/5 months of the year. If they employ a gardener or street cleaner then this also has to be paid for plus the lighting in communial areas.
The other thing that hasn't been mentioned yet is health insurance. Are you going to take out private cover?
All I can suggest is that you read all the different message boards and then ask questions.