Can you not just put in graphics card(s) that will allows the number of monitors you wish to use. Buy the Flat Screens from wherever you wish, then use some monitor software to give you the layouts you want?
I am sure I had a link to some multi monitor software, I will have a look and get back if I can find it.
How do you route the keyboard and mouse to talk to the correct screen?
Is it possible to have say 8 screens and say 2 keyboards/mice, with one set of inputs talking to say 4 screens and the other inputs talking to the other outputs?