Welcome to MP3car, way to jump in with both feet!
I have a couple of comments on your planned spec's:
"I feel I will need 2 PSUs (1 front dash for 2x Monitors and 1 rear for the computer)"
You should not need a second PS for the monitors, they normally come with their own filter and can connect directly to 12vdc.
The system will be running Windows 7 x64 as I have 2 available licenses for it
x64 does apparently create some issues in certain instances with Centrafuse (if you are going that route) I am not aware of other software issues with it specifically but others may chime in.
1)I am not well qualified to comment on this question, I run a laptop and I am perfectly satisfied with it's speed.
2)Most monitors are LED backlit now. There are lots of threads here to search, you will find the biggest issue is sunlight readability on the cheap ones.
3)CF is the most polished front end (IMO) but comes with major heratache from bugs and quirks. RR is catching up fast, it's definitely worth comparing for you needs. OM is new but making great strides as well.
Hope this helps.