The chances are, John, that they won't know if you don't declare ---- they will, however, ask for details of your car and will wonder where the money came from as it will have recently been purchased. However, even in this scenario, most decent IP's will take the sensible view that a reliable vehicle, especially if needed for work is a reasonable investment, as it will, in all probability, save money in the long run. Here, I am talking reasonable amounts for a reasonable car -- don't look through an Aston Martin showroom window!
I downgraded, a few months before my IVA, from a petrol guzzling QX (25 to the gallon and £200 plus road fund licence) to a frugal diesel Skoda Fabia (Affectionately known as "The Skip") (50 to the gallon and £95 RFL). My IP said nothing, however there was mention in my proposal that I had switched to a more frugal vehicle in an attempt to minimise ongoing running costs, but no figures were mentioned.
I also, as you mention, stashed my months unpaid payments in an emergency fund, as, although RFL and repairs are built in with a small allowance, of course it takes time for this pot to build and sod's law dictates that the breakdown will not wait!
The bottom line is to be sensible (i.e don't splash thousands) and be honest with your IP --- he won't be impressed if a nest egg of a few grand comes out of the woodwork during the 5 years ( and these things do have a habit of doing so!).
My opinions are merely that .. opinions based on experience. Always seek professional advice.
IVA Completed 23rd July 2013 .... C.C. 10th January 2014