Well it would be best to use a cheap wired only router.
That is what I would do. Especially if you don't have a switch already. No point in buying a switch to find that it doesn't work when a router is generally not much more and will work.
I've tried the modem -> switch -> multiple computers and it didn't work with my ISP. I contacted them and they stated that they only provide a single IP address and if I wanted to connect more I would have to pay extra.
A router fixes this. The router has a WAN port which will get the IP address from your ISP, then it will use DHCP to distribute internal addresses to your machines.