can I just pick up a cheap PC to run iVMS 4200 on to record 12 cameras at 3MP and have live view on screen??
Nope... it will melt
Very beefier will be required, or a mid range pc may cope with all the cams throttled back to maybe 720p (or less) and low frame rates.
IMHO .. a Hikvision nvr 16 channel + hard drives, viewed and controlled via a monitor is the answer, (or reduce the requirements)
P.S.. vga / hdmi can be sent via extension or cat cables if the nvr has to be located in a different part of the building from the viewing area, and mouse control over distance isn't rocket science