1) Xenarc 700 series going to a Lilliput touchscreen? A screen going to another screen? What's that all about? You need a Lilliput or a Xenarc, but not both.
2) The amp's rating should be based on the RMS rating of your speakers. You want the RMS output of the amp to be within the RMS rating of the speakers. For optimal perfomrance, you want the RMS rating of the amp to be as close to the upper limit of the RMS range of the speakers. For the four 50w speakers you specify, a 200w (50x4) would do nicely.
3) Take out the switch between the batter & the Opus. The Opus has a startup/shutdown controllwer so you don't need a power switch (although you can still wire one up if you choose).
4) There's an ATX power cable bundle that goes from the Opus PSU to the motherboard. IT looks just like the ATX cable in any other ATX power supply.
It looks like a good starting point.