Hi
this is, or could be, a very complex task; I think it is too wide and complex to be addressed in a forum. I suggest you to start making a baseline of the network, without a deep and complete understanding of the new network you can't design and plan anything. After this you'll decide where to start, and how to proceed with LAN and WAN integration. Planning and design an integration is almost impossibile ignoring how this network is done, which kind of hardware and software it use, switch and router configurations, technology used, routing protocols and so on.
I don't know books on this very specific task, but a book I liked very much, in its second edition, was "Top-Down Network Design"; if memory serves me, it has also speak about network integration.
By
enrico