To your earlier question... Yes, get the GNSS sensor installed in the new AHARS package so your EFIS will be capable of fully stand-alone flight planning in the event of failure of your IFD440.
As for a backup battery, it may be wise to consider installing an Essential bus or similar, powering the second power input of your EFIS and AHARS as well as, perhaps, your IFD. The Essential Bus will take its power from a circuit breaker on the main bus, through a diode, to the Essential Bus and it's associated circuit breakers. A small, independent battery can be tied to the Essential Bus through a robust switch or via a small solenoid. In this manner the Essential Bus is normally powered by the Main Bus, with the Essential battery available to provide backup power in the event of a failure of the main bus.
In operation, one switches on the Essential Bus Battery and powers up the EFIS and GPS equipment prior to engine start - this allows all these boxes to get aligned, get positions, and perhaps get a flight plan added. The Essential Bus Battery then protects the equipment from "brown out" during engine start and gives you full engine instrumentation right through the engine start cycle.
In the event of a failure of the Main Bus (alternator failed and main battery discharged, main battery failed, master contactor failed, etc) the Essential Bus Battery will continue to power your essential equipment until it becomes depleted.
The above power distribution design is fully supported by the GRT equipment, thanks to their units having three individual power inputs. The IFD would be powered directly from the Essential Bus.
I have our aircraft wired in this fashion and it works a charm. Of course we also have a Mini-X installed and, with its internal battery option we will have reliable Primary Flight Display and VFR flight planning available for a long time. In fact with some manual shedding of high-current loads (pitot heat, landing lights, strobes), we can sustain an alternator failure shortly after takeoff and still have electrons flowing after burning off all of the 50 gallons of fuel on board.