Details on Di2 Groupset
AggieRay74 ........
I bought an upgrade kit from Planet Cyclery. The cost was $1,148. It was their part number GSX1040-1-PMFRB (2013 Shimano Ultegra 6770 Di2 Upgrade Kit Internal). You can see what is included on their website at http://www.planetcyclery.com/group-sets/2013-shimano-ultegra-6770-di2-upgrade-kit-internal/ . It looks like the price now is $1,195. In addition to this kit, you will need the crank, brake calipers, rear cog, chain (if you are using the chain stay extender, you will need extra chain), etc, etc. I bought an additional 1000mm cable from bikebling.com for $34.99. Di2 IS a little pricey. Whether it is worth the price is a matter of opinion.
I like the Di2, but don't expect too much. In the past I have used lower end shifters, Shimano Ultegra shifters, and SRAM force shifters. Properly adjusted they were all excellent, and I could tell little difference in performance. None caused me to go faster or slower. The one thing I have come to like on the Di2 is that I can shift from just about any hand position. On mechanical shifters going from smaller to larger cogs requires a little more effort. With the Di2, I can shift with my little finger from any hand position. No....... I did not drop $1K to be able to shift with my little fingers. For me it was just an experiment to see what the big deal on Di2 was all about. Ultegra Di2 has a 3-position auto trim feature that I like. People have mentioned on the Cruzbike website that it really isn't required. Still, I like this feature. Di2 is easy to adjust, and supposedly requires less maintenance. Properly done, it makes for a very clean installation. There is a lot of good information online regarding installation and adjustment of Di2.
As John Tolhurst has pointed out, you don't need a chain stay extension to mount Di2. Whatever means you decide for mounting the battery pack, I recommend you have a solid plan before you begin your build. Otherwise you may be rebuilding parts of your bike several times. The standard Di2 battery mount is designed to mate via water bottle screws (check this out online, if you haven't already). The battery mounts to a bar that ends up in close proximity to the bottom bracket of a standard diamond frame bicycle. You will not find such an easy means of attaching the battery mounting bar on the Silvio. Getting a clean installation on the Silvio will require some planning, and frame modifications (hole in the boom by the bottom bracket and extending the bracket that holds the forward derailleur at a minimum).
Good luck on your build.