hi
afaik i dont think your 805 will be able to accomodate 3 connections as per your requirement here (Serial link/LAN/Internet).
you need to go for an upgrade so that you can accomodate all the three connecitvities onto a single box.
I would suggest to check out for Cisco 1841 series router with Wic-1T card where you can terminate your serial link onto it and 2 fast ethernet ports available for your LAN and for internet connectivity.
regds