I think Rivendell will install the BB and headset if you order them with the frame, maybe there is a small fee. With the BB and headset installed you don’t need a ‘vendor’ to build the bike.
You can assemble a good set of hex keys, a cable cutter, decent pliers and adjustable wrench, a cassette tool for your cassette, a chain whip, crank puller, and tire levers. These are the basic bike tools any home bike mechanic should have. If you go slowly and carefully you can put all the parts on yourself in a couple of weekends.