Not trying to discourage you, but getting a straight chainline could be a challenge since many (all?) DIY mid-drives have chainrings that are further from the frame than normal in order to provide enough clearance for the motor from the frame. Some DIY'ers use a Lekkie chainring, which moves the ring in substantially closer to the frame. I built a SS, but had a cassette rear hub so there was plenty of room to move the cog around and get a straight chainline.