[werk_ag]

I don't see an SMA connector on the TX board--how is that antenna connected to the board?

There are already some info about this on the forum. You may need a small SMA pigtail. To connect the SMA pigtail GND signal, you have to carefully remove some of green varnish, near the Ant hole on the FS1000A module.

I also see that some of the Arduino Nano options on eBay (clones, no doubt) come without the pins installed. Obviously the pins on either side of the boards will need to be present, but does the 6-pin header need to be installed? It's not a big deal either way, but I just don't see that those pins connect to anything.

The 6-pin header (or ICSP header) is not needed.

Right now my eBay cart is at about $125US including shipping. That should include all the parts I need, as well as some spares (large-ish capacitor, resistor, and LED assortments, the 7809s are in a 5-pack, etc.), but doesn't have anything in the way of enclosures. Seem reasonable?

The answer to this question depends on the assessment that each will do, considering the pros and cons of WeatherDuino Pro2 system.
The final price of the system can also vary widely, depending on the construction options and the sensors used. On the other hand, many people may have at home, some of the components such as resistors, capacitors, transistors, etc., etc. The system uses only parts with common values.

Certainly I'm suspicious to talk about this, but I truly believe that considering the long time run cost of a weather station, not be dependent of a brand for replacement parts, is a considerable vantage.
What to say about the Wireless Display Units? You can have many as you wish, all over the house, and each one will not cost more than 35€ to build.

Please spend some time reading the forum, it will help you to better understand the whole system.