At op free power = good.
gpu prices = bad
what will you be paying for those gpus's
6 x 700 = 4200 too much
6 x 400 = 2400 may work out.
I can not find a cheap 5700
used for 850 and no warranty
https://www.ebay.com/itm/353527825310?hash=item524fe6839e%3Ag%3AAbQAAOSwfxRgv-xf&LH_BIN=1I got one in jan from amazon new with a warranty for 399
I picked a random ebay seller I do not know who he is
but 850 a card = you lose
400 a card = you may do okay