slowster wrote:….All that's deterring me now is the cost of the hubs and the fact that I would have to dismantle the rear hub first in order to fit the longer HSA808 axle, because my frame is 135mm OLN.
Yes, a SA 3s hub is quite able to tolerate low gearing and it will remain comparably reliable. Plenty of AWs have survived well in tandem use, although SA don't recommend it.
HSA808 axle will work with an NIG 3s hub. In pre-NIG 3s hubs, I don't think you can quite get to 135mm OLN unless the dropouts are very skinny and you put the NTWs inside the dropouts too.
Maybe you are thinking about rim brakes in which case the wheel is an easy build. But if you are thinking of hub brakes don't forget that SA do rear brake hubs for derailleur gears too; X-RD for screw-on freewheel, and X-RDC for 8/9/10s cassette.
I might have a good used X-RD3 hub here, and I could quite easily fit it with the longer axle etc if that sort of thing isn't your cup of tea.