Actually if you are going to do most of your own work BMWs are not more expensive than most other cars. This is especially true for the e30 318 and 325. Of course I am a bit biased having owned several and currently racing an ITS 325, but I have not driven a better street car for certain and the race car has to be one of the best. One thing you have to do is get the information CDs which are available on ebay that break down every part on the car, repair instructions, wiring diagrams, etc. You will be very far ahead with this. Also, there is a very strong BMW community on the web with several very knowledgeable mailing lists in addition to bulletin boards. All in all the enthusiast following will be a big help in for you and your son. It is what I plan to do, of course my son is only 4 and BMW doesn't sell a kart yet. Check out the BMW CCA club racing site, there are a number of e30 cars there that should fit the bill:

<http://www.bmwccaclubracing.com/forsale/forsale.htm>

Feel free to email me for other sources, lists, etc. Best of luck to you.