I think you are on the right track. Likely need to use both Serial and USB for your setup.
Yes, serial conversion gets a bit confusing. Here’s good info on why it’s required…
https://www.sparkfun.com/tutorials/215
Adapter you linked should be OK for UART/TTL to RS232 for your Reach to ag equipment interface. Then use USB to UART for the Reach to RFD900 connect. Use something like this to convert from Reach to radio:
You can supply power in via the other DF13 on Reach. Just don’t power your Reach twice.
I have not tried the RFD900’s. Heard good things. For best results be careful of fresnel zone for your base antenna location. You will likely need to get that radio/antenna pretty high to clear fresnel zone. E.g. for a 5mile/8km link you need to be at least 50ft/15m over highest obstacle. Here’s a calculator (they give 80% but I would think 60% is acceptable for 900MHz). It will work if you do not, but not as well.
http://www.proxim.com/products/knowledge-center/calculations/calculations-fresnel-clearance-zone