Just get the 12v adapter off ebay, think I payed $20-30 for it. Don't see why you would need an OPUS, but maybe that's just me.
I originally was going with a similar setup. Dell CPTV with that dock. I just abandoned it (last week) as the dock audio doesn't work under linux and never noticed the normal headphone jack was missing the Left audio channel. . I even tried cutting out the speakers and going with a speaker to line circuit, but still not good enough, so I got a new dell (Inspirion 8600)
Here's detailed pics on modding the PDX docking station from when I did it.
There's also a smaller dock (port replicator) if you don't want the PCI slots.
If you are using the PDX, why not just get a PCI sound card?
If you have an internal modem and your BIOS supports it, probably could go with "Wake on Ring" to turn it on instead of hacking the power button, mine had the option, but not the modem. I bought one but if you don't get it from the factory, the solder pads aren't there on the motherboard
The only hard part of getting the laptop to start is you can't just jump out the button, has to be a somewhat short pulse. (At least on mine). If you simulate holding the button down by just jumping it out, it reboots over and over, just like if you held down the actual power button. You can use an RC circuit like the lilliput auto on that's been posted here over and over.
Also, shutdown is easy, there's an option for turning it off X minutes when it goes to battery, so when you remove power, takes care of itself.