I am certain these questions have been asked and answered a number of times in the past but I couldn't find anything within a couple years so..., I would appreciate some advice and thank you in advance.

This will be my first wheel build. I am in MN and laid off so I literally have 2 months to build these.
Shimano HB-6207 hubs and Velocity Deep Vs.
Spoke calculater says 283.5 in front
282.2 left rear
280.6 right rear

My questions are...
Round up or down and how far? (will 282 work for front and left rear, will 280 work for all)
Should I use a heavier spoke on the drive side? (DT revs front and left rear with comps on the right rear)
Brass or aluminum nipples?
Best place to buy spokes and nipples?