Buy a 16 port 10/100/1000 switch and connect all the devices to it via ethernet. If you are running local lan games you should have a full gigabit between all machines. If you are running internet based games the internet connection is your bottleneck. In general the router is not real critical since you only have 100mbit internet. Most routers can easily do that. I would still buy one with gigabit wan and lan ports just because the cpu in those routers is more powerful also.
Other than that you need nothing real special. I would lock the machines down so nobody can install any software but you. You might want to install some kind of bandwidth control app but as long as people can not install software it will prevent most abuse. You don't want to let some idiot kid load bit torrent and overload you internet connection.