I'm planning on adding DCS to my Legacy setup once the full blown iOS app is released. I think I'm going to get the TIU/Handheld 50-1001 Set and the WIU 50-1034. I think with the app all I'd really need is the TIU and WIU but I like the idea of being able to isolate things for troubleshooting with just the TIU and the DCS remote. I think the only other part I need will be cable MTH 50-1032. (I have both the 2nd and 3rd editions of Barry's book and on page 126 of the most recent 3rd edition update it shows either the 50-1007 or 50-1032 cable. I'm guessing cable 50-1032 would be the preference if you have the choice but that's just a guess based on their respective catalog appearance dates on the MTH website.)
In my opinion, one of the most overlooked benefits of the Lionel LCS-WiFi module was the ability to update the 990 Base and the Cab2 via the LSU without any serial/USB cable between the Windows machine running the LSU and the 990 Base. This was a particular plus for me given that a) my Macbook Pro doesn't have a serial port and b) I was running Windows XP via VMWare Fusion on my MacBook Pro.
I'm curious as to whether with the WIU I'll be able to update the TIU in the future in a similar way that avoids any physical connection between the Windows PC (or emulated Windows PC on my MacBook Pro) and the TIU. Or perhaps utilizes the iOS app to avoid the need for a Windows PC at all. I plan on buying Barry's new WIU book as soon as it's released but I'm really interested if MTH is planning on having this sort of update capability for the TIU/WIU combo.