If they were only able to afford £250 per month, then they would need to inject at least £14,000 from an equity release at the end of the arrangement to effect even the minimum dividend likely to be accepted by creditors - and as they have £50k in their property they are likely to be required to pay a lot more.
Their disposable income should be based on their own individual circumstances, and you will need to get them to put together a detailed income and expenditure summary together. I have a form on my website which you might find useful.
Regards, Melanie Giles, Insolvency Practitioner for over 20 years.
For further details contact me at
http://www.melaniegiles.com and view my IVA blog at:
http://melaniegiles.blogs.iva.co.uk