The expansion kits will not work as effective as you believe. With an expansion kit there are 3-4 slots available but currently issues with it and you will only have 2 slots working. So in reality with an Asus z270-a you can put 7 expansion boards and 2 m.2 to achieve 16 GPU's but you are going to have many issues to try and save a little by doing that. Your better off getting a high PCIE board and building multiple units to not worry, especially if you are building this for a house as you would need 30 amps atleast, or a 240V plug. Best off getting 2 750W Psu an Asus z2700-a and running 9 cards on thatm, and building 2 systems for 18 GPU's.