Jokes aside, Mailcow works very well and the author's support is awesome; this one I use and recommend.
I would say that Mailcheap's web site has lots of information, but that's me. I don't use it as I host my own email (besides using Office 365/G Suite/others depending on the client). I just seemed to me that it checked all @thwr needs while being recommended by people I trust.
The free license is limited to five locks per day which means the free edition defends your system against five unique attacks per day. [...] The free license does not contain reporting (like the PRO edition does).
Also, no official support for Windows Server 2016.
1U and 2U commonly have "single server" prices. Once in a while you can get 3U and 4U prices but after 2U you normally go for a fractional rack. 3U+ servers aren't often useful for hosting. Not many are made and those that are typically aren't very dense.
If you're looking for something similarly spec'd you could just buy and beef up a Thinkserver TS440. They are pretty inexpensive, have 8 bays for drives, Xeon E3-1225 v3 proc, ECC memory, redundant power supply (only comes with on slot populated), and start off under $500.