Jim,
RickO is correct about the preferred wiring type, however many people have added DCS to their existing Legacy Layout without rewiring. Give it a try 1st and see what happens, with the DCS. A lot of this depends on how large your Legacy Layout happens to be, when you want to add DCS to it. There is lots to learn about DCS, pick up Barry's DCS O Gauge Companion and the OGR Video Guide to DCS, both these 2 formats have great educational material in them.
I run DCS and Legacy and have a great time doing it, make sure when you purchase your DCS it has an L revision TIU and is up to the latest changes. I have no need for WiFi so I can not help you there, if you need more help come back and the guys will help you blend the DCS into your Legacy operation.
Good luck and have fun.
PCRR/Dave
Better get started, Christmas is just around the corner!