I don't think it matters whether you wire white as hot and black as ground, or vice versa. The black and white wires are the two ends of the pickup coil, the bare wire is to ground the chassis of the pickup. So just make sure the bare/screen wire, plus one of the others, goes to ground, and the other wire goes to the hot lug of the pot. And wire both pickups the same way or they'll be put of phase with each other.
The other two questions are a matter of taste. P-90s are single-coils so you could argue the "conventional" pot value would be 250k, but they're often used with 500k. I read recently that old Gibson Juniors had a 500k volume and 250k tone, but I've no idea whether they were wired that way as standard or whether they've varied it over the years. Apparently the current ones have 300k volume and 500k tone, like most new Les Pauls. I'm going to try the 500k vol/250k tone setup next time I rewire one of my Juniors.
Basically 500k will give you a slightly brighter tone, if that's what you like.
For the capacitors, the higher the value the more treble it will roll off. If you don't like the neck pickup to get dark with the tone control turned down, try 0.015, but I'd probably start with 0.022 and see how it goes.
