1. As above, smallest you can get
2. As above, Dosn't matter.
3. I am a poor solderer. My skills leave a lot to be desired. I used solder with flux built into it, however I also used a tub of flux and put a dab on each point to be soldered.
All flux is for is to remove any oxidisation which would otherwise prevent the solder from sticking to the joint. You only need a tiny bit but that tiny bit really does make a difference IMO.
Its up to you.
If you're "shit-hot" at soldering then dont bother, but if you suck, like me, invest in some.
It will make life easier.