Need a keyboard recommendation

[EricT43]

I am interested in getting a keyboard for home use, but I don't know very much about them and would be happy to get some recommendations or pointers. Here is a list of my needs and wants:

- use it for my 5-yr-old son to learn and practice piano
- I might want to learn how to play piano myself at some point
- I want weighted keys to simulate a piano feel
- I will use it to aid myself in learning music theory, listening to how chords sound, and things like that
- I want to hook it up to my computer, initially to generate decent sounds for Band In A Box, but in the future I see myself perhaps composing, sequencing, and doing all the incredibly cool stuff that a keyboard can do in conjunction with a PC
- I would like to not spend more than a few hundred bucks for a used one if that is possible for what I want to do with it

Any help is appreciated. Also, anything I need to know about the keyboard/PC interface would be helpful.