That is a really cool system, bit bad timing though 
but you already knew that.
But it will still be pretty sick with 4xP4 in it. Do full-height cards not fit in that case?
I just finished my plotter and although I also like this as a hobby, cash is also a thing. Also for me big part of the hobby is to get it done on a budget.
I got a system now with a 1070ti, 256GB ddr3 and dual 2630L v2. Tested it yesterday with both BB and MM, plots are around 4.75 minutes. Total cost just shy of 500,- euro
I only have 22 drives to replot and just finished my new DIY JBOD’s. .
My plan is to just hook those up with the sas port and copy to the drives from a 1TB buffer nvme. By copying to alternate drives in sequence it should be able to keep up with plotting even though it’s SAS II