Nope, it wont matter when you last used the credit card, though obviously you should stop using them right away as it is time to address your problems rather than compound them.

It could be a slightly different story if you recently obtained a loan, as some creditors dont like a IVA being proposed within 12 months of a loan being given.
In BR, regarding your car, it would be upto the OR as to what he deems is acceptable evidence. If there was a statement that showed an amount coming to you, which then left at the same time you bought your car that may be good enough. As long as everything adds up, I doubt the OR would question it and if your dad puts it in writing that the money was given to you for this purpose, then I would think there can be little doubt. But I'm only speculating on other peoples experiences that I've read about...