In most cases at home, WAN or LAN NIC cards, its all the same. Just get yourself at least two nice compatible Intel NICs (can be on a single card).
Not sure about you second question, but you can get gigabit throughput pretty cheap. Possibly even rummaging in a junk yard for hardware. haha.
Sure - You can get your DHCP IPs from your modem, no problem in most cases.