And I wanted some advce on building it. The case will probably be wooden or some thing simple and flexible from random crap I can find around the house, the real problem will probably be the screen though.

Currently, I have chosen this for the battery just out if simplicity, but it may not work for the project.

For keyboard I'm looking at this and this. I'd like to have the raspberry pi running RISC OS for speed/performance purposes and because I'm a lot better with BASIC than Python, but if there would be a problem with connecting the keyboards to RISC OS, are there any simple fixes to it, or would I be better off going with a linux distro (raspasin, I think would be a good one)?

For the screen I'd like to use the same one used in the pi to go, right here but I'm not sure how to exactly connect it, and I believe it requires an external power supply, which could be a challenge. If I need to use a different power supply than the one mentioned above, I'd like to keep it at around $40-ish.

I think that's all I have as far as questions, any advice for my build would be great though.