I could see W&H as a form of ordered evil not entirely different from The Peacekeepers or Star Wars Empire, in as much as their true pursuit was their own power, exploiting whatever they came across in order to increase that powerbase

That's a darn good point; W&H are very organized corporate evil, they want their "i"s dotted and their "t"s crossed and everything filed in triplicate. It's an organization built to feed on and exploit existing human weaknesses, and as such it largely autopilots itself, absorbing whatever it can, eventually including Angel and his team. In that way, it does have quite a bit in common with The First - as liliaeth said, what we actually see onscreen suggests that it was manipulating Buffy into doing its work for it, to taint and corrupt the greatest number of people. The only place The First seems to veer off that path is when it reveals an eleventh-hour motive of wanting to "feel" - wanting to make the transition from being the puppet master in the shadows, ala Wolfram & Hart, to something that could experience every bloody act directly. So maybe that's the key - to emphasize that difference, between the detachment of W&H (Peacekeeper-like) and the visceral, emotional (Scarran-like), internal threat of The First, where W&H want to let humans turn their own world into hell on earth that they would then look on and file reports over, and The First as wanting to execute a full-scale Exorcist-style possession of all of humanity... getting in moreon the ground floor, so to speak.