LOL! Sorry! I did not mean to disparage your older hardware; if you can still use it, I applaud you and your willingness to not be wasteful.
I myself am still rocking a loaded and modified 2008 Mac Pro; I almost made it to Mojave, but they definitely took out enough code that my older Xeon will not handle it, even though I have a fast modern video card capable of Metal support. I’ve been grasping at it about three years longer than I would like, but of course, there is no new Mac Pro yet. And I still don’t know if I will buy what they do produce because I’m afraid it’s going to be too courageous.
And like most good Mac users, I too, have an attic and basement full of past and glorious hardware. My beloved G4 dualie was acting as a server until very recently. I also still have my oddball NuBus G3 7100, which is awesome to keep around for booting System 7 and Mac OS 8 and 9; and the floppy and Zip drives and SCSI are all very handy on both it and the Dualie; and there’s still a loaded SE in a nice travel case that gets pulled out once in awhile.
What’s really incredible is that Apple File Sharing still works with High Sierra to the old rigs.
I’ve been on the hunt for a dead Luxo iMac; I want to gut it and install a Mini or a Nuk to play media server. May or may not try to replace the terrible LCD with an IPS panel.
Anyway, unless you’re doing big media at home that is bandwidth constricted, stick with CAT 6e (or plain CAT 6); it’s often cheaper than CAT 5e because it’s more widely available and is worth using. Just be sure to use the appropriate jacket for your run, be it plenum or riser cable; from what you described I’ll assume you need riser cable in your walls; but if you’re planning on being inside a duct inside the wall to make it easy, or inside a drop ceiling in a basement that acts as a plenum, technically you should use plenum and junction to riser. But I’m also going to assume no one is ever going to code inspect your install, so riser it away because it’s usually cheaper and has a better bend rating.