I've retro-fitted Di2 to both my road bikes and have a set of R8070 Ultegra Di2 bits ready to fit to a new frameset when I choose what I want
I originally fitted Di2 to the best bike but liked it so much I fitted it to the winter bike too as I missed it when I went back to mechanical...
I own both and would say there's no noticeable difference in use between R8050 Ultegra and R9150 Dura Ace so save your money
There's not a great deal between R8070 and R6870 in performance either, but the newer R8050 gets the shadow rear mech which is less prone to damage and much nicer hydraulic STI's if you have disc brakes (which I do).
To convert a non-Di2 bike that already has a Shimano 2x11 groupset you'll need:
- STI's (shifters/brake levers), generally the most expensive part, especially if hydraulic
- Front Mech
- Rear Mech
- Battery (several options, make sure it's compatible if you want to use a wireless module)
- Junction Box A (an electrical hub that the wires go to at the front of the bike) - I use the EW-RS910 as it allows internal cables and it super neat
- Junction Box B (same but goes near the bottom bracket)
- several eTube wires (these are about £17 each and you'll need between 5 and 8 of them depending how you connect it all up, don't underestimate the cost of these!)
- Wireless module if you want to connect to the system via bluetooth
- Charger
- Di2 fitting kit for your particular frame, grommets, battery mounting kit etc..
The costs all add up but IMO it's a worthy upgrade if you're using the bike a lot and have the money available, I wouldn't go back to mechanical on my nice bikes now although I still use SRAM Rival on the commuter