Given the SH-2(I) avionics are now themselves a decade old are they much of an upgrade over the avioincs available in the SH-2G(NZ)?
The RAN Seasprite has (had?) higher-spec sensors than the NZ Seasprite: the RAN Seasprite sensors were first-rate 15 years ago, but at best would be considered middle-aged if not "mature" by now. Still supportable, but probably due for an upgrade. Presumably the Seasprites being considered by MoD have not been upgraded and this is partly why they are so cheap.
Let's see now:
Radar: RAN Seasprite has the APS-143B V3 Ocean Eye with ISAR, NZ has the baseline APS-143 V3. The APS-143B is still serving in limited numbers: Denmark (in 3 Challenger MPA), USCG (in 6 HU-25 Falcon jets, due to be retired 2014, + a couple of Metroliners & other odds & sods). It was also picked for the Swedish NH-90 (presumably the 5 maritime versions), which are still a twinkle in someone's eye. The RAN Seasprite appears to have been the largest customer of the APS-143B version [1]. The USCG HC-144A (CN-235) uses the later APS-143C version, and MH-60R uses the more advanced APS-147 development.
FLIR: RAN Seasprite has the AAQ-27, NZ the AAQ-22. The AAQ-27 is used in the RAN S-70B Seahawk & USMC MV-22 Osprey. So support for that probably won't be a problem.
ESM/ELINT: RAN Seasprite has the AES-210E from Israel. Used in the RAN S-70B Seahawks as well, so that would be a potential source of spares when they are retired.
[1] My source for this is getting a bit old (The Naval Institute Guide to World Naval Weapon Systems 5th ed (2006)), so I may be out of date here.
Chis73