Hi
I feel u can lookout for something inline with 1700 series like 1721.
In that u got to have 2 Slots where in u can install 2 wan interfaces one for the primary serial line and the other one for the backup isdn.
check this link out for more info on 1721.also you got to have a onboard fastethernet port in that which is also one of the requirment u have out there.
http://cisco.com/en/US/products/hw/routers/ps221/products_data_sheet09186a00800920ec.html
These are the interface cards which u require for 2Mb link connectivity and for the isdn connectivity.
WIC-1T -- For 2 Mbps Link
WIC-1B-S/T -- For ISDN backup
Also you can check out for WIC-2T in which u will have 2 Wan interfaces and will have the reduandcy on the hardware side in that for the WAN interface.
regds