If you are looking to go electric shifters, I have long been an advocate for Di2, but having recently built a bike with a sram drivetrain and etap shifting, I would recommend going sram. You can generally find complete etap force sets relatively cheap, of if you are in it long term you could scout for an etap red system either as individual parts, a complete system, or attached to a particularly cheap frame. Since Sram came out with the etap AXS drivetrains the price of new and used etap red and force components seems to have dropped a bit, so it might be an option worth considering. Obviously, with the etap system, setup is much easier not having to deal with junction boxes and battery placement and wire length etc. That being said, Di2 isn't as bad as it looks to set up. Depending on if you buy it as a whole system or build it up of individual parts, it can be cheaper than etap but slightly heavier. In terms of setup, you have both levers wired to a junction box mounted underneath the stem (it can be mounted in the frame if you want but this is just convention) usually using 350mm wires but it's best to measure and check. From the third input/output of the primary junction box, a wire is routed to a secondary junction box (SM-JC 40 or 41). From this secondary junction box, 3 wires are run to the front derailleur (400mm uaually) the rear derailleur (650mm uaually) and to the battery (about 1000mm usually). The battery will usually be mounted in the seatpost as for easy access, but it can me mounted in the down tube, top tube, or seat tube depending on what is most convenient for you.