If I understand it correctly the FX-10 was/is primarily an OEM device. The FX-05 was first supplied to OEM's (Reznor used a ton of them) and "those" OEM's would not release (make public) the the apd files to controls people.
As long as the equipment supplier will guarantee that you, as the "Control Guy", will get fulll access to the program files logic I think you will be OK.
I , personally, would make dang certain that I had top notch support from the Manufactuers Controls team on the Heat Pumps. The OEM probably has a confidence level with the FX-10 and their control logic so they certainly have people that have "seen it all" with respect to field weirdness.
I might think twice about N2 but the LON functionality Sucks (IMO) on the FX produce line. BACnet might be an option if you can get it that way. Easier to integrate than N2. Also easier to replace if/when N2 products are no longer available.
s2sam you said "JCI is discontinuing the smaller FX product line (FX10 and downward and possible the FX20?)". The FX-20 (JACE200) is a Supervisory Device not a field device. It doesn't make sense to to me that it would be discontined, where did you hear that ?