Yeah. After butting some hours of research into this I'm afraid that would be a little too big project for me to start with currently. So from this site ( http://www.fentek-ind.com/FootPedal.htm#afs3usb ) I found a little idea which i just might try out. But instead of leg switches, I thought about adding a new modifier(s) under/near my palm rests (pressure switches I guess, but needs some testing first, what feels comfortable) to get brackets, colons, <> and other daily signs from the home row.

For keycaps, you can often find mechanical keyboards at goodwill. Failing that, there are many companies that will sell keycap sets to you (Like WASD)

For the shells, yes, those will cost a bit of money. Or you could make friends at your local hackerspace.