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Thanks for the reply! I have about a thousand tabs open right now. Should also do some actual work, but oh well.
It'll be a second bike so no hurry. Doing some renovation at the moment plus a baby incoming so bike building will be slow. I have no spare parts lying around so might as well buy the entire groupset. Or a frame and swap everything. Argh, should read up more.
mtsdw-
Does also depend on the build. Both my Di2 bikes were built up using individually sourced parts, which is a pain but I just didn't need all the parts in a groupset for either bike. There should be a list of parts you need in this thread. Basically with Di2 it's shifters, Junc A, Junc B, FD, RD, battery, cables. Mechanical is that minus junction boxes. Then you get into specifics like are you doing 1x? Are you using cable or hydro brakes? Which wheels are you using - what thru axle size and disc rotor type do you need?
hippy
Generally it's cheaper to buy the whole groupset but sometimes, ie. if you already own some of the parts, it makes more sense to buy individual components. Some people buy entire bikes and do groupset swaps then sell off the other frame. Really depends how much time you have, how much space you have and exactly which parts you need for the build.