Hi and welcome to the forum.
All IVA companies will say that they may be able to write off a portion of your debts but there is no guarantee and the portion varies from person to person.
All of your disposable income after priority bills have been paid will go towards the IVA but you are allowed a contingency fund to pay for any emergencies.
I'm surprised that you have been told to change to a repayment mortgage - it's usually the othrr way round.
I would recommend that you speak to one or two more companies. Visit
www.iva.co.uk and speak to some of them. The advice is free and impartial.
Sharing from experiences of dealing with debt
The greatness of a man is not in how much wealth he acquires, but in his integrity and his ability to affect those around him positively.
Bob Marley.
http://kallis3.blogs.iva.co.uk