Can you use one of the 4 switch ports that have 100 Mbps for your WAN connection? I am pretty sure you can just want to be sure as I believe the 800 series specifically the 831 has only 10 Mpbs WAN port.
Based on the spec sheet, the 851 has a 10/100 WAN interface:
However, having a FastEthernet interface doesn't necessarily mean the router can handle that much traffic. The following sheet is old (2009) but gives an idea of relative performance under specified test conditions.
On the 881 you cannot change the switchports to routed ports (and I assume the same is true for the 850). You can set up VLAN SVIs for routing, and that may or may not work for you depending on what functions and services you plan to use. Not quite the designed concept, but it may work.
For ethernet WAN, useas minimum Cisco 851 or 871 or newer. Other models are really really old.