Maximum length is purely about the ANSI/TIA/EIA specification, which for both Cat5e and Cat6 is 100 meters per link/segment, so a little bit more than 300 feet actually. 300 feet is just easy to remember and gives you some headroom.
If you need to go more than 300 feet, then you'll need some sort of active repeating device like a switch, and since this is for AVB audio, that means you'll need an AVB switch, like this: https://www.presonus.com/products/SW5E
That's 100m per link, so 100m max from the 32R to the AVB switch, and then you can go another 100m max from the switch to the SL32 Console.
Cat6 will be future proofed a little bit, but that's really only important in the case of a permanent in-wall install. If that's the case here, sure, may as well go Cat6. If there's a big price difference between Cat6 and Cat5e, Cat5e is fine if you want to save a little money. Just depends on how much cable you're running, how much you care to future proof, and what your budget is vs. the price difference.
Don't get older plain Cat5 though. It's not officially rated for gigabit speed.
