The plan is to thread up the wheels and hand them over to a builder for tensioning and finishing.
I am curious why people do this. Does it save money or do you not have a truing stand? The lacing is the trivial part; building the wheel is the tensioning and truing.
Can I use the ERD measurement supplied by the manufacturer directly in a spoke length calculator, or do I need to measure myself with a nipple in the spoke hole?
You really want to measure them yourself, and since you have to order the spokes separately anyway, there is no cost savings here to be had by ordering all at once. Personally I would recommend using the round polyax nipple washers too, assuming they fit in the rim bed. Juat provides a nice interface for the nipples, though i am sure it will work without them (assuming they do not recommend some other washers be used). The polyax washers add 1.5mm (total) to the ERD.
Also, suggestions on spoke pattern and spokes are welcome. The rims are 20/24h, the hubs are BHS SLF85W and SL210.
Radial front and 2x/2x or 2x/radial (ds/nds) would be the standard lacing patterns here. 3x probably won't work with 24 spokes, and no reason to deviate from the standard IMO. Slight stiffness advantage to radial NDS in rear, but 2x/2x is what I would do for overall wheel strength.
You don't have to use cx ray spokes. If you are not doing deep rims for aero -- and I assume you aren't since Farsports doesn't have data (to my knowledge) to show their rims are more or less aero than any other (e.g. shallower) rims -- then any round spoke will be just fine too. Sapim Laser, for example -- especially since you aren't doing the build
(cx-ray are easier to build with since you can easily see, and hence avoid, windup)