Sunil,
I would recommend you to get the 2800s.
BRI requirements: You will have to get a min. of 2811 or higher to support NM-4B-S/T.(infact 2801s does not have any NM slot)
Ethernet requirments: Note that all 2800s have 2 onboard ethernet ports. You can get a HWIC-4ESW which is 4 port switch wic that goes in the wic slot. 2811, 2821 and 2851 all have 4 WIC slots.
Serial port requirements - With this router basically you have only 3 more WIC slots left. You could buy WIC-2T's (2 nos) and use 4 external CSU/DSU's to meet your requirements. Otherwise, if you are looking for a built in CSU/DSU, you will need to get WIC-1DSU-T1-V2, but only 3 can fit in the router.
At this point your 2800 series router will not be scalable enough to add more NM's or WIC's, which is when you want to look for a router like 3800 series.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus