Questions question! Thats how you learn grasshopper!
Not familiar enough with that engine to know, but I would think you could use 4.8v or 6v. I use 4.8v 1800ma on my DL50. And yes, you do need a switch for the ignition and a charge jack so you dont have to remove the battery. You can use a big receiver pack with the standard receiver connector (servo kind of connector). As far as size of battery, that would depend on how much current the ignition draws, my guess is its not too much so the 2000ma you have in mind should work fine. And 1500ma for the rec/servo should be ok for a cub. I separate my receiver and servo power into two different batteries. Only because I use spektrum. This helps reduce low voltage reset. Spektrums have been known to do that. Its never happened to me, just extra insurance and piece of mind on my part. I normally use 1400ma to 1800ma on my old 72mhz systems. My yak has three switches, receiver 4.8v 1800ma, servos 6v 3600ma, and ignition 4.8v 1800ma.
I keep spare plugs, but have never needed or used one. So, your call.
Fuel dots. Thats what you plug your fuel fill line with. You can get nice aluminum or cheap plastic from dubro. Either works. I use a two klunk setup.
Kill switches are used on magneto engines. Remember a few posts back? You dont need it for an ignition engine. BUT, its a real good idea to have multiple ways of killing your engine. You have the battery switch for the ignition, you can set your throttle linkage up to close completely and kill the engine, you can have a parallel switch controlled by a servo (in parallel to the ignition switch). Something to consider, I set my throttle up so that the pushrod moves to the rear of the plane to reduce throttle. This is just in case your engine comes loose from the firewall. If its the other way, as the engine lunges forward the engine surges. I've had an engine come loose just enough to not be able to kill it or control throttle. Had to raise gear and plow it into the grass to keep from totally destroying the plane. Beat the cowl up pretty bad but damage was repairable. This is just my opinon.
Fuel jugs. I like one gallon. Easier to mix, faster to go through, less chance of getting stale.
Fuel filters, I dont use them. Depends on your environment. If its real dusty, might be a good idea. Put it where you can get to it easily. Just an opinion, I wouldnt know.
Tygon, think I covered that further up.
Building boards are to each his own. Lots of ways to do it, whatever works for you.

Somebody else jump in here to give different opinions.
Edwin