Having a rack is a massive QOL improvement IMO, I originally had a stack of rackmount servers in an IKEA Lack (LackRack), and it was okay, but the rack is so much nicer.
The only thing I wish I had done was get an enclosed rack to help with noise and dust, rather than just a startech 4 post from Amazon.
And that's a lot of awesome stuff for free!
That seems really resource heavy. 4 Gb of RAM for an SMTP relay?
Other than that it looks pretty great.