Unfortunately you have a situation that I think many enthusiasts find themselves. It looks very tempting to buy high end control equipment from eBay, but it's not as easy as it may seem to set up and program. I noticed that quite a few of the auctions end up re-listed.
I very much doubt if any installers here would provide the software as we have spent time many hours in classrooms and on site learning to set up the hardware and program systems.
In order to set up the system you have you will need:
- A fundamental understanding of the RS232 protocol (plenty on the internet)
- Device RS232 protocols (some manufacturers will provide them to the public)
- Soldering skills (get a 'helping hands' with magnifying glass)
- Cat5 cable will do to practise connections
- 9 pin male and female D-type connectors
With the above you should be able to control the Onkyo and Plasma from the PC. You should be able to view feedback from the Sky box by setting your port to 57600,8,NONE,1,NONE you will see a pattern in the feedback which can be parsed for usable touchpanel feedback. There are time stamps, programme descriptions etc in there.
I'm afraid when it comes to the AMX programming, unless you have programmed in a language such as C before then your going to struggle big style. Even if you have, without the fundamental AMX courses under your belt it is a very steep learning curve. There is a reason programmers charge £350+ a day, and it's not greed
To give you an idea of program structure I've attached a very old bit of AMX code I did after completing AMX I about 4 years ago.