RE: Recommendations: RC electric car/buggy/truck with $400 budget and following criteria
well, 3 cheap kits that spring to mind are all tamiya's, the df02, the dt02 and the lunchbox. all 3 come with a basic motor and speed control so all you need is a battery, charger radio system and servo. none of these are quite what you'd call quick but they are plenty fast for a beginner. when more speed is desired all 3 can take the power of brushless and they are cheap enough that your budget could actually swing 2! many dads here will tell you to get 2 because you will be fighting your son for the controller.
the [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTDR5&P=7]df02[/link] has gone up in price recently but is still the cheapest 4wd i know of. i personally have owned 2. they have no real issue with brushelss other than the normal hickups. stock, it'll do 15-20 depending on the battery which is actually faster than it sounds.
the [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXMVP3&P=7]dt02[/link] comes in a few different forms but the one in the link is more or less the top model. from what i've seen, it's a very simple build.
then there's the classic [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXKSG2&P=7]lunchbox[/link]. it was originally released in the 80's then re-released several years ago. it's a fairly simple design but that just means there is less to break. it's rear end is shared with several other models (mostly buggies) that may be of interest as well.
as i said, all 3 come with esc's and motors so all that's needed is a battery, charger, servo and radio system.
if that seems overwhelming, [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXCYBG**&P=0]BAM![/link] [link=http://www.hobbypartz.com/thac6smbachw.html]shizam![/link]