1) You won't get 5V at the board because the cable will have some resistance and drop some voltage. So if you put 5V in, you might get 4.9V out, but that depends on the cable. However, if all you care about is getting >3.6V to drive an LDO, that's no problem: Your cable just needs to be less than (5V - 3.6V) / 80mA / 30m =
0.58?/m. Just about anything meets that, including cheap fine gauge ethernet wire. Do make sure you have a fair amount of capacitance on the field board, however, as that long wire isn't going to be able to respond very quickly to changes in loading.
2) It will depend on you signaling rate, of course, but supposing you are under ~100kbps you won't likely have any issues. TI has a
design guide, but if it's just point-to-point like that RS485 is pretty simple.
3) Unshielded twisted pair is the standard choice here, with one pair for power and one for data. If it was me for a hobby project I'd probably use Cat 5 ethernet wire as it's cheap and would work fine. If you're in an industrial environment, you'd probably want an overall shield to help control noise. Size isn't likely to be an issue, but I'd recommend 24AWG / 0.2mm^2, give or take a little.