The news today regarding the tightening up on benefits looks pretty grim for anyone who is claiming at the moment.
I would have thought that the best people to talk to regarding overseas payment of benefits is DWP at Newcastle.
If you do a search you will find the contact details, telephone etc. Others have reported that they are very helpful.
Are your parents retired or nearing retirement age?
If not are they reliant on the benefits they are receiving now as an income in Spain? Have they worked out an annual budget yet of how much it will cost to run their apartment.
Can they speak Spanish, are there any local clubs in the area they have bought? Some areas have pensioners clubs and it might be helpful to join one to make friends with the Spanish as well as the English.
Where we live the Spanish teenagers are very noisy, they drive up and down the street on their motobikes. They've gotten into trouble mainly due to boredom, their latest trick is to throw shoes over the telephone wires and grafitti our walls (according to our presidente)
Before they sell up why not suggest they spend a long period at the apartment to see how they fare, best to try the Summer and Winter periods. If they are still keen to relocate afterwards then finance permitting they should go ahead.