DI2 on a triplet should be no problem. You might consider using the Ultegra version at this point due to the simpler wiring. The Ultgera (and 2013 Dura-Ace) use a bus type of wiring so that any wire can be plugged into any open port and everything communicates. This will allow you to easily connect multiple wires together with a simple junction box to make the long wire runs you will need. These junction boxes can be attached to the frame. If you want wider gear ratios with a double you can do the rear derailleur cage mod by K-edge and run compact cranks. UI2 shifts are amazing.