Ex-FSO
I only answered one of your questions:
1. As I noted previously, some avionics manufacture advertise things as using the same tray - inferring slide-in slide-out, but in fact they require a rear connector change and sometimes even a change to the avioincs locking device (ie through screw)
2. WAAS GPS needs not only a new antennae but also new cabling. I THINK that the WAAS GPS antennae is slightly larger too. I would't guarantee that the WAAS one will be bolt in.
3. ADSB requires a C146a GPS input (ie WAAS) and does not allow C129a input. As Dick has noted, this is partly because CASA are taking a very purist technical approach.
4. ADSB requires a "grey scale" output altitude encoder. Unless you altitude encoder is relatively new; you will need one of those too. This will probably require new bracketry and additional wiring and may trigger a static system test.
One of the problems with the CASA regulations is that (unlike the FAA) there is no definition of "minor modifications". Therefore, strictly speaking, everything requires an EO.