Bah, tank first then house! First rule of reef club? Tank comes first! Second rule of reef club... Tank comes first!
1: I wouldn't fixate so much on an actual amount of rock, but more so what rock you'll need to have the structures that you want. I've seen some of the TBS rock, compared to Pacific rock, it stinks (ok Pacific rock that's been sitting on a tarmac in 110F Las Vegas weather stinks too), but eh... is all I can say.
I wouldn't suggest a group buy on rock since you typically pay per pound and shipping is per pound as well, so the only cost you'll be splitting up is any sort of boxing charge which usually is minimal compared to the other cost. Only reason I'd say group buy is for a small tank, where you can order a 90lbs box of Fiji or something and can't use it all so you split it.
2: Debate? schedule 80 comes in pretty dark greys, 40 in white. They are interchangable with each other, other than maybe a bulkhead there's no real need for the additional strength.
3: Pricey skimmer, I think over at RDO this talk went about as well, where for the money you could do with a lot more skimmer performance for the same price as BK.
4: Man I forgot how deep your tank was... lumenbrights are more parabolic and are great for focusing downward more so than the lumenarcs, if you need to punch through the water then great, if you want it more to spread out evenly then the later is probably better. No opinion on the ballasts, and your LED actinics, fine for when the halides are off, however I don't think there's anything out there that really can make a difference in halides like tradition actinics can de-yellow lights quite well if that's the effect you're going for.