I am not so sure that all Respironics machines with Cflex are data data capable. I if I wanted cflex I would have the script say so and if I wanted a machine that collects "useful data" I would specify that machine. If you are considering Respironics then the easiest way is to eliminate the "plus" machines regular or m series and the regular Pro. The others all have data M series pro, regular Pro2, auto, and auto m series.
About faxing stuff to a DME. I also think it is better to get it right then fax it.
And, there is no reason if you know what machine will be prescribed that you can't give them insurance info and have them price it for you. In fact you can do this with multiple DME's at the same time.
You don't know until you do that if the insurance coverage is worth anything at all. It is dependent on your deductable, and your copay as well as the level of machine the DME is trying to give you. Figure your copay on their base price and compare the exact same machine to cash price on Cpap.com.