Hi, and welcome.
Firstly, overtime isn't taken into account when initially looking at an IVA as it is not guaranteed income. Your payments will be decided on your normal disposable income after priority payments have been made.
If you work overtime, you are normally allowed to keep the first 10% over and above your normal wages and then 50% of the remainder.
If you have a car on finance, you are normally allowed to keep that as well, but you have to be able to fund those payments outside the IVA and if your finance finishes during the IVA, those payments get added to your IVA payments.
CAB are not the best ones to speak to regarding an IVA as they are not experts.
I recommend that you visit
www.iva.com for a list of companies and reviews. Give one or two a ring and you will receive free and impartial advice as to all the options open to you and the best one for your circumstances.
The forum is also a great place for information and we have professionals post on here as well.