Do not know about your local avionic paperwork needed, but recently we done facelift of one old fix wing airframe with:
G5+G5+GNS750+GMU11+GAD29B+TT31
Now altitude encoding goes
-over RS232 from G5 to TT31 with full ADSB out
-over CANBus to GAD and than via Airinc 429 to GNS750
Total cost level of $30 k USD