If they're going to be running any Windows OS, then forget the idea of reasonable boot-up times. I'd be far more inclined to use the 'standby' or 'hibernate' features.
Standby would be the fastest option, though whether it's practical would depend on what sort of current these devices pull when in standby mode. You'd need to weigh this up against the amp hour rating of the car battery and try to work out whether or not the car would still start if the devices were left on standby for a few days.
PSU-wise, this looks interesting: http://www.mpegbox.com/dsatx.html
It claims to pull 4mA when 'shut down' and 70mA when operating with no load. I'd expect the standby current to be somewhere slightly above the 70mA figure. If you're budgeting for a PSU for each unit, then that'd be a tad over 2 amps constant current draw with the machines on standby, though it might be possible to run multiple systems off one supply. At 2 amps, you'd drain an 80Ah battery down to less than half capacity in 48 hours, so that's certainly a worry.
I reckon I'd go for using hibernate, and trigger it from the car's central locking. That way the systems will automatically shut down when you lock the car, and will immediately start to wake up as soon as you unlock it. With luck, they'll be fully operational by the time you've parked yourself in the driver's seat
Edit: Just a thought, but for the rear multimedia systems, it might be quicker cheaper and easier (why does this remind me of an advert) to strap one of these to each headrest: http://www.3wisemonkeys.co.uk/proddet.jsp?id=1137&cat=1