This is difficult to answer without knowing the whole scenario and application. What speeds? How many interfaces? Do you need voice or other ALG? Firewall? VPN? Dynamic routing protocols? NAT? MPLS? IPv6?
As far as eliminating collisions, make sure that your ethernet interfaces can be configured to negotiate full-duplex. Just about any router made in the last decade should handle this. "Allow packet flow", that's kind of a given. You'll be subject to whatever shaping is in place on the connected interface, of course.
Much more information is needed. From what you've given, you could be asking for a box with a dozen 10G interfaces capable of multiple full tables or a home NAT box running at DSL speeds.
