I think you could get used MUGEN MBX5R now that the Mugen MBX6's are hitting the shelves next month. I would wait one month and scour Ebay for used Mugens. The MBX5R would be a great first buggy used or not. Also check out the RB sport motors like the S3 and S5, those are budget race motors that will last through the learning curve.

If it must be new.. this is the best bang for the buck RTR buggy with motor.
http://www.airlandhobbies.com/catalo...7370259739.htm

If you can spend just a bit more here is the pro version.
http://www.airlandhobbies.com/catalo...7300195055.htm

I would recommend saving until he has enough to purchase a Losi RTR or race roller, this is a big step up and more of a real race buggy. Here is a sale I found on the race roller, not sure how long it will last...
http://www.airlandhobbies.com/pages/index.htm