I have used 4321s with the NIM-16A or NIM-24A, but based on the price of these, it is becoming increasingly difficult to justify them over purchasing a dedicated console server from another supplier.
Hope this helps,
I think You can bay another 2511 on ebay :-)
Other way - use small PC and PCI-X card with 16 RS-232 but it have high cost.
get a 2811 or 2911 with a NM-16A or NM-32A with the octopus cables.
Cheap solution should cost you around $100-$150 on ebay