Screens are the most expensive item. You need to figure that out first.
Minimum new would be an D510 Atom motherboard/cpu $80, case $40, PS $30?, and ram $25. I don't think I would tear apart a laptop unless it was near free.
A KVM can switch between front and rear, depending on how you run your system. You will need to figure out multi screen, especially with a budget mobo.