err... I would dare asking one question, but multiple ones in one thread makes it look like your lazy and want others to do all your thinking for you. However, I do have some time on my hands
there are already posts on o-rings. I remember one of the top of my head from BigBen2k. Do a search. To find out what screws or whatever to use, consider land space in which you have to attach, and what screw you want to use or have room for. If you provide a screw, there is no reason they cant tell you what bit to use. So the question is how deep of material are you attempting to use, and that determins what screw head to use. Circular logic, but it works. Go to a metal shop and ask to see some screws. Say.... "that one" and they will cut u a hole. There is a chart floating arond here on hole specs. It has green, red and little arrows. Ill let someone else link you to it.
Did you consider bringing your mobo to the shop and letting them figure it out? Get a ruler.