Just an update:
We fitted the Ryan 9900 BX TCAS, with its stand-alone indicator and voice alert system with mute and refresh button on the collective. Provisions for future expansion for lightning alert and EGPWS.
Absolutely marvellous, and the altitude alert functions and approach modes work like a charm. They would want to, coming with a price tag over $55,000. But a lot cheaper than bumping into somebody.
Heck of a time finding somewhere to put it on the dash. Eventually had to toss the ancient Garmin 165 and shuffle a few dials around. Still got two other GPS to help me not get lost, so the 165 can be a spare for the Huey.