Actually, depending on where you live fleas can always be a problem. Here they are really bad this year, even though it has been so cold. If you have these cats trapped, you can buy the full strength dog advantage and use it on them in measured dosages. It is cheaper to do it this way, and i have done it for years on my ferals with no problems. You do have to know how much the cat weighs, so what I do, is I weigh an empty trap, write down the weight, then I trap, then I weigh, then I deduct the weight of the trap and I have the weight of the feral (approx)
I would get some area spray from your vet, and once the transport is completed you can spray out the interior of your car. I also use borax powder in the carpet, let it sit a few days and then vacuum. Also, I wouldn't de-flea until after the visit, some of the ferals may not survive the initial exam (if you are testing for diseases) which you really should. For those who are healthy, they would be the ones I would deflea.
Let me find the website that talks about the advantage dosehttp://www.lisaviolet.com/cathouse/advantage.html