Usually the cost is in running the wires, so if you're running them already adding a few more won't bump the cost much.
I suggest running more wire and conduit (if you're running conduit) than you plan to use.
- If running conduit, and running 1, run 2 or 3 so you can add more power or data lines in the future.
- Get 2x 30AMP, one on each 120v leg. (So you can add more equipment later, and if needed swap in case one side has issues and you need to keep something running.) This also gives you the ability to re-use some of these wires in the future for a single 50amp breaker\220v outlet should you update\change your PDU\BBU. (I try to make sure to put it NOT on the same leg as your 120v refrig or other leg with motor going on\off -- you can't avoid it 100%, but the fridge is usually it in the home that goes on\off... most new ACs are inverter \ not huge load on startup anymore so that's good)
- Run 1x 15 or 20amp 120v -- Use this for lights in\around your rack, and add a plug near the rack so you can use this for lights \ other misc in front of rack, and also in case you have breaker issue with the larger you could use this for equip you want on NOW (ie: your router and POE gear)
If you want to add a "kill switch" the cheapest way is going to be adding an AC Disconnect, they'll come per-packaged with what you need, and come in various sizes. Like an AC you'd want to place this within view of your rack so you can hit it quick and easy, so not behind cabinets, cupboards, not behind the boat always in the garage you can't get to, etc...