Personally, I would always go with the router method vs. the ad-hoc method.
Pros of routers: NAT protection (better than nothing, but not bullet proof by any means), inbuilt DHCP services, network management options aplenty, room for future growth, etc. A decent Amiga TCP/IP stack will support DHCP so hopefully it's as easy as plug and play, for the most part. Using a link to a laptop requires the laptop being present, noodling around setting up IP addresses for the ad-hoc connection, etc. Last consumer grade router I bought was about $35 and worked a treat with my old A1200 back then, but I've switched to bigger iron routers since (Cisco in a rack). Was completely plug and play and didn't require another machine being on and present. If you go the router method, make sure you have admin privs on the WAN side disabled for security - and always change the inbuilt, default logon credentials (logon name, password) even if WAN side admin rights are disabled as they should be.
You mention you have several routers laying about, minus power bricks. Can always take a peek on the back of said routers, note the voltage and amperage required (look it up on the manufacturers website if you need to), and you should be able to pick up a power supply brick for a few bucks at a Radio Shack type store. On the consumer end of things, I've always been a fan of the WRT style routers.
As for cables, a straight through cable will be used from a router to a PC. A Crossover cable will be needed for switch to switch type connections. Both will need to be CAT5 (8 wire/4 pair) rated for good measure.
Good luck!
