Damn, probably the worst thing Avaya did to the IP office was allow BT to have their own branded versions (hell just allowing BT to install them is a joke). We have taken over the maintenance of so many BT installed IP Offices as they install the same config into each site with the most basic of changes incorporated, so instead of a phone having a persons name on it, it will have User 201 etc, then they want to charge more to put real names on it.
Have to agree with others you should definitely be looking at SIP over ISDN, it is about half the cost just for the trunks, then calls are cheaper and if you need to move office you can just move without having to get the lines and telephone numbers moved.
Drop me a trust message if you ever get really annoyed with BT and I am sure my company can do a much better job (we were the first company in the world to install and IP Office in Japan and have other companies all over the world).
For the router, personally I would be looking at getting something like a sonicwall in there and have both the fibre connections running through it, either in load balance or auto failover mode.
Finally I will say best of luck.