all I will say is I still have the Original Di2 DA setup on one of my road bikes - 10 speed. Last year when I was having an issue and needed to buy some replacement parts to try and figure out the problem......parts were available, but not by any means in abundance......even scarce I might say.

So, to answer your question, yes - you could probably put together 10 speed Di2, but getting replacement parts if something gets damaged......may not happen overnight!

Don't worry about the wheels, go for 11 speed Di2. You can get a cassette machined to fit a 10-speed freehub body (check on Google). You can also use a Shimano 11-34 11-speed cassette on a 10-speed body.

Yes, ...given the right price. Di2 is great and the benefit is most noticeable to me on a tt bike.

I have been able to get some excellent deals on used 10 speed Di2 equipment (since 'everyone' wants 11 speed) and I am really pleased with those groups.

I've also learned that ultegra 10 speed groupsets can be reasonably upgraded to 11 speed by swapping out (upgrading) the derailleurs/cassette. The rest of the electronics automatically function as 11 speed.

10 speed Ultegra Di2 is great. DA 10 speed is bad. They are totally different designs. Many of the 10 speed Ultegra parts are compatible with current gen 11 speed.

Technically you can use Etap on a 10 speed cassette. It's just an 11 speed cassette with a cog removed. I did that for most of last season.

But then you have to deal with where to put the blip box.

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Strava