Hi Adam and welcome to the forum
I don't know how thee "IVA wizard" works, but you have to pay in your full disposable income to an IVA, as a detailed list of your income and expenditure needs to be submitted to creditors as part of the proposals. So if your disposable income is £1,000, then this is what you can expect to pay.
An IVA can be set up fairly quickly - within a month if necessary, and your employer does not need to know. You can keep your Amex card, so long as it is paid off every month.
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