Hi Pete, welcome back to the scene.

Most people will either use multiple packs or perhaps two packs in series, running the higher voltage components off the two packs but taking a feed from just one pack to power the lower voltage components. This has a downside in that one battery will be under more load than the other; if you're using lithium polymer packs, you'd need to make sure it doesn't lead to over-discharging. No-one really uses step-down electronics to get different voltages but that's not to say it wouldn't be worth exploring.

If going the multiple packs route, to get through a 3-minute fight with a standard drill motor drive setup, you don't need a large capacity pack. Perhaps 1Ah to see you comfortably through. Upshot of this is that the pack will be physically quite small so it won't take up too much room in your internal layout.

As for spares, I'd always go for a minimum of one spare pack for each voltage arrangement, but if you have the budget for more, go for it, as it takes a bit of pressure off in the pits if you've got several charged packs just ready to drop in.

P.S. I believe Martin Dawson is meeting with you soon to collect parts from Milly-Ann Bug. It's myself that's got the Milly chassis now, so hi and thanks for passing the bits on
P.P.S. I'm not sure if you'll be aware or not, but there are featherweight events that take place in Sherburn-In-Elmet throughout the year (full combat and 'sportsman', aka no spinners) so if you're looking for events to compete in, they'll be reasonably local for you. Nice relaxed atmosphere at them. Next one is March 22nd if you wanted to pop along and get a feel for the current field.