Hello Nikhil,
you need a router able to host 16 E1 / serial links and then to build a PPP multilink with 16 links.
I would go to C7200VXR with NPE-G1 or NPE-G2 and two 8E1 Port Adapters.
You probably can be able to install 16 serial interfaces on a C3845 but I'm not sure it can be able to handle the 16 links bundle.
The best suggestion here is to ask for subrate 32 Mbps service provided over a FE or GE link it is far better then having so many links in a PPP bundle
You could even think to move to a L2 MPLS VPN service to get the desired BW.
An E3 link (34 Mbps) can suit your needs too and an NM-E3 can be found for C3825 or C3845.
Hope to help
Giuseppe