10.4" Touchscreen : $270 (http://cgi.ebay.com/10-4-TFT-LCD-Tou...QQcmdZViewItem)
8" TS Monitor for the back (x2): $175 each (http://cgi.ebay.com/TM-868-8-USB-Tou...QQcmdZViewItem)
All the other stuff would require you to do some custom programming. No one is going to do it for you and hand it to you on a silver platter. From the nature of the "how much for this stuff" question, I assume you just want to pay thousands of dollars and get what is on that list. Not going to happen plain and simple.
You will have to do a lot of it yourself. And I would definately reccommend against an electronic handbrake. Computers fail, and that would be bad when driving down the highway.
Engine start is doable and is easier with a remote start unit that has an aux in such as some of the Audiovox units for the Satellite pager. It starts whenever a ground is received on the aux bus. So then you just need to control a serial port to send out a ground on a button press and use a relay to make it only go out when the ignition is on.
To run your stereo you need a $3 cable that goes from 1/8" jack to RCA's to go to your 4channel amp which will then go to your speakers. Computers unlike a HU cannot go directly to speakers so they need an amplification source like an amp!
Climate control is doable probably serial again but it depends how your climate control is wired. Going to require custom circuit boards. Not good with Electrical Engineering? Then don't try it or you'll fry your climate controls.
For the air bag suspension, there is an entire thread out there tited something like "control air bags with PC" or something. Search for it. I think the descision last time I read it was to get a pre-built unit and then interface it with the PC basically use the PC as the remote for the unit. So it goes your finger on the PC to the unit to the air bags.
To control your windows again serial output.
For something like this if you are not willing to do it for yourself I would say about $800 for the 3 monitors and fabbing them in. A PC that is capable of doing all this probably $1000. Custom circuit boards and software $2000 for reliability. Wires and everything else probably $200.