I've done what you are describing, I re-used the crank, bottom bracket and (rim) brakes, cassette and chain. I had to purchase shifters, F & R derailers, battery (and seatpost shim) and battery charger, A junction, B junction and 7 E-Tube cables of assorted sizes. This system generally runs about $1100. I found Texas Cyclesport to be the best deal at the time (3 years ago) and they did a good job of packaging to fit.
I also did this on a year old Chinese carbon frame designed for Di2 and internal mechanical cable routing and that's the 1st question - is your frame set up for internal Di2 ?. Like others I would not spend this money on an external system based on esthetic reasons alone, forgetting that Di2 is a superior shifting system (IMO) and has required zero maintenance.
Would I do this on a 6 yr. old frame ? - No. Is your current bike 10 spd. ?, that would maybe mean a new crank, bottom bracket, cassette and chain, so add at least $200 to the budget. Is your rear wheel 11 spd. capable ?, not all are.
Begs the question that a certain level of mechanical aptitude is required to get this installed and set up, and you have issues now keeping a mechanical Ultegra operational ?. Do you really want to tackle this ?.
Good site here for add'l info.
https://di2center.com/