[werk_ag]

Hi, danb35

Welcome aboard.

• The lists for each board identify two Arduinos in the Designation column (ARD1, ARD2), but give a quantity of 1. Looking at the board images, I only see room for one. Am I correct in understanding that it's one Arduino Nano per board?
  

Yes, just one Arduino Nano per board. ARD1 and ARD2 refers to each side of Arduino pins (the schematics are not very well done )

• Similarly, the RX and WD board parts lists give designations for two RX modules (RF1 and RF2), but list a quantity of 1. Never mind; I see that RF1 and RF2 on these boards are for the two sets of four pins on the RX module.
  

Right.

• The TX and WD board parts lists specify 1% metal film resistors. Is there a particular reason that this degree of accuracy is specified? A 5% carbon resistor is much more "standard".
  

5% carbon resistors will be fine. (Sometime I will change this on the parts list).

• This post identifies this RF module set as working well and being a dependable seller. To build all three boards, I'd need two sets of these (1 TX module for the TX board, 1 RX and 1 TX module for the RX board, and 1 RX module for the WD board), right?
  

You will need one set for the RX board, and just a receiver for the WD board. The TX v2.50 board uses a different transmiter module, the FS1000A (the receiver in the bundle is a trash, don't use it).
With some small changes on the connections, you can buy just two sets, and use that small TX module with the TX v2.50 board. It just don't fit directly in the board, but it will work well.

• What about antennas? If my count is right for the RF modules above, I'll need four antennas as well, right? Are there any particular characteristics I should look for, or just get something simple like this?
  

Those work great for me!

Doesn't look like you have many of these running in the U.S.--should be fun to add one to the number!

That's true. Only three units in the US (one recently sent to Havai). I don't know if they're ever built or online, so far I did not get any feedback from these users.