I have been on the same path as you - see post here: Proliant DL380 g6 Server slow plotting - #24 by teeboy - some of the G7 servers I talk about are much lower spec than you were looking at and really low price!
I have sold most of the servers now - my theory is for < £100 you can get a server that will do a plot per hour in memory and save to cheap disks. This is less money than a single SSD drive. Only use Madmax on the servers, then move the plots to a cheap farming PC.