Near as I can tell, all the leaf packs come from National Spring. The AC ones say "Rancho" but I can't see any difference in the construction except for the two load leafs which are pretty much useless. You really won't save much money between dealers on the 10-pak.
Some people will have different opinions, but I don't see anything wrong with putting your system together a piece at a time. It let's you "feel" what each upgrade is giving you, and you can cut some costs here and there.
As far as the differential lowering that SLR does. It's just an added safety factor to reduce the chance of blowing a CV joint. If you ignore the diff kit, there is nothing extra that has to be done to install a SLR kit. They just provide it, as they think it's worth doing from their testing.
I think all CV joints that have blown were on vehicles with the front sway bar disconnected (and some other offroad factors).