I want to find the lowest end (most cost effective!!)
You might want to give a try to GNS3: http://www.gns3.net/ before to buy hardware devices.
"GNS3 is a graphical network simulator that allows simulation of complex networks."
"...Connection of the simulated network to the real world!"
You can load inside the same IOS images as your actual production routers to be as close as possible of your production network. The following tutorial gives further details: