englishal & andrewc
I can see that TCAS must be an eye-opener and I also know (simply from flying under an RIS) that there are many more planes around which one never sees (even when told roughly where they are).
The HUGE problem is the number of people who fly without a transponder, or with it switched off, accidentally or deliberately. This is immediately apparent when getting an RIS. Many that one does spot are far from being bits of old junk.
I would not spend any money on a box which does not tell the direction in which to look. Most of the alarms one will not be able to act on at all.
So I could spend about £25k on kit which would help by perhaps only 30%. The other 70% won't show up.