My mum had one put in when my dad was ill. They bought one - possibly a reconditioned one, I'm not sure. They were told the company would buy it back when they'd finished with it. But when it came to have it removed the company had a surplus, and so didn't particularly want it back. So with the removal charges my mum didn't actually get much back at the end.
In retrospect they agreed they would have been better off going with a 'hire' scheme like the one MonkeybirdisboredWITHbuns mentions.
It depends what the plans for the future are I suppose. Is it reasonable/likely that your parents will continue to live in that house for many years?