1) ICS : Internet Connection Sharing
2) Router would resolve that situation completely, since your computers are connected to the router via ethernet cable or wireless (depending on the type of router and what you have for hardware on the computers for the connection.)
Since you're already using ICS, then you already have what is required for a wired router, with the exception of the router itself.
Most companies that make routers have good to excellent customer service, and there's also this site
which has a lot of configuration information for multiple types of routers.
3) Sorry, can't help with that one.
4) Go to mIRC Options - Connect - Options - Advanced
Ensure that DCC is checked
Set the number in the box marked First to 1050
Set the number in the box marked Last to 1059
(Using the example port numbers)