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I have the seatpost battery and the handlebar mounted gear indicator (which Im assuming is the junction box), the shifter is Shimano's own alfine gear and brake lever
this one
http://www.artscyclery.com/Shimano_Alfine_S705_Di2_Shifters/descpage-SHDI2SLALF.htmlwhich most seem to say isn't compatible..
I have no idea what a clutched mech is..
I have no idea whether a narrow wide chainring will fit the crankset I'll use on the bike...
Would like to reuse as many components where possible..
Yes it would work fine with 6870, 9070 or xt and xtr di2. You might want to go for a clutched mech for a 1x set up.
Do you have internal battery and a stem mounted junction box?