You only need on Legacy command base per your layout. I have 3 TIIUs on my layout One of the TIUs connects to the Legacy base (separate cable), and the TIUs talk to each other. Barry's book is mentioned above, and that is a must have and will address everything you want, need and more.
Yes, you will also need one WIU for each TIU you have. I would not view this as "resigning yourself" rather freeing yourself. No more "engine not on track" or other messages that prevented you from running your trains easily. To me the MTH app is easily one of the best things to happen to this hobby in a long time.