Peter,
Check out Bendix/King EGPWS from USD 12K up to 40K for the box. Be aware that you need something to display the data on which is optional. Garmin 530 is or is about to be TAWS B compliant. Upgrade existing box is about USD8K.
If you have a glass room the display can be easier but be aware that avionics people rarely use a common language.
Possibly the smartest thing I have seen recently is enhanced vision systems. Low cost, low weight (2.5kg), and the real picture long before the Mark 1 kicks in. Uses a microbolometer (uncooled IR). No I don't work for them, it just looks very promising. Get your glass box to look at this with your AI and FD over the top as a layer. Gulfstream G550 has something similar using a HUD.
Gulfstream EVS
Max Viz