You can do this with pfSense but there are some things you should be aware of:
1. Not all USB broadband devices are supported, check the forum.
2. pfSense will ping the gateway of each interface continually to check that it's still up. This could eat into your data allowance if you have one so you might need to disable it.
3. pfSense is a complete operating system designed to be installed on separate hardware so when you say:
@positron:
I have no need for other users on my LAN to access the internet in the fail over scenario. So, I don't need to bother with a separate PC with multiple NICs
you will in fact need either dedicated hardware or run it virtualised.
Steve