Fortunately there is a little more help on this one. Check any or all of these 
places:
 - As mentioned above, stores often offer package deals. If a package deal comes 
with a motor, it will often be matched fairly well for that car.
 - Once you decide on a car kit that you want, check the manual that comes with it. 
Stores often have a copy on hand, or you might be able to get a copy on the 
manufacturer's web page (see "Links to links to manufacturers and stores" below). 
The manufacturer will often recommend a motor for their kit.
 - If you intend to enter into sponsored races, a motor is usually one of the items 
that might be restricted - you are allowed only certain motors. Check with the 
organizer of the race.
 - If you did post to the newsgroup asking for a car recommendation, ask for a 
motor suggestion as well. Again, expect several (sometimes contradictory) answers.

What you will end up with probably, is a some sort of a spec for a motor. See "What 
about motors and stuff?" below, for an explanation of what this spec means. Go to 
your LHS with this spec, and ask the salesperson to show you one of those.