Almost all routers can handle gigabit, which is almost certainly what you want if you plan on doing local networking. A typical hard drive has speeds of about a gigabit. There is no reason to get anything slower. You can also get some gigabit switches (or even faster if you are using nvme on both machines) and connect two machines that need fast speeds between them to it. Most switches will be able to send packets to each other without going through the router.

If you really want to do some learning you could try to set up an opnsense box on an old PC and connect that to a switch. It’s feature rich and completely modular and upgradable. This is probably the best thing you could do if you want to learn something but also the worst thing to do if you want consistent uptime since you can pretty easily break stuff if you don’t read the docs.

That said, as others have mentioned openwrt on a used router is probably the best of both worlds - feature rich but less breakable.