Changing arb's affects the balance of the car, in stock form they are pretty understeery, stiffen the rear arm and you dial some of that out so def worth doing that, but your only transferring the flex to the drop links, so I'd advise uprating the drop links.
It's then a case of how far or more to the point how much do you want to spend?
Anti lift kit, arbs, drop links, other bushes, coilovets etc is pretty much the full shebang.
Personally I don't rate bc coilovers at all, seen to many people moaning about them. Last car I went in with them they were pretty harsh ride, however different spring rates will affect that and products do decline over time so the newest ones could be a much better setup.
Whiteline stuff is good, but I find it starts looking a bit tatty pretty quickly, although it doesn't affect how they perform and other than the looks isn't anything to worry about.
I've got tein flex z coilovers, perrin rear roll bar, perrin drop links and have to say I've been mega impressed with the setup. It's firm but not crashy or too harsh a ride for normal road use.