You'll need a special USB cable ("EW-EC300") to charge the Di2 when using the 105 derailleur. It is
not included with said derailleur and costs € 40+
Use the b-screw to get the upper pulley wheel quite close to the 39t cog (vertically speaking), I think that the fact that I can't because of the restrictions in the velomobile adds to the clunkyness.
I'm using Rotor 30/46t oval chainrings. I like the 30/46t size but feel the oval-ness adds to the chance of dropping the chain when shifting from big to small chainring (again, having a chain that is 3x normal length with an additional idler does not help chain tension).
Shop around, I found a cheap address in France for the Rotor cassette, then used the "price guarantee" of bike24.com to get a good price to combine with the rest of the parts I needed.