A brief update (er, if anyone is interested):

We discovered that my old Dell Latitude was not in such bad repair as first thought, and I have spent the better part of the previous evening and this morning reinstalling WinXP on it.

There is space on my desk for K to work directly next to me. So although it is a laptop (which will be convenient for travel), it will function essentially as a desktop for him, and I will be able to supervise easily.

My next step will be finding software to help manage his internet usage, as I expect that most of the software he will be using will be online. I want him to be able to access information on and pictures of trains and rabbits, his two favorite things, without accidentally stumbling onto something he can't unsee.