Firstly, I would say going without a landline would be a mistake. Reason for this is that whilst it will technically work, wifi/3G/etc is always going to be less reliable than a landline and you will spend a not-insignificant amount of extra time setting up, managing, and troubleshooting such a connection.
That said, a Draytek router (2830n) which would probably be useful for you as it allows you to choose between ADSL, Cable(Ethernet) and 3G as 3 forms of WAN connection, and would allow you to change to a landline later on if things change. The 3G is via USB port (so takes a USB 3G dongle) so the Mifi may need swapping for a USB stick instead (but at least the Draytek can cope with more than 5 clients unlike the Mifi).
It also allows you to set up OpenDNS or use it's own built-in keyword filtering.
As regards USB DVD drives, these should work with no issue on XP.
Any problems, give me a shout on PM.