This is the master reference thread for Lenovo Tiny https://forums.servethehome.com/index.php?threads/lenovo-thinkcentre-thinkstation-tiny-project-tinyminimicro-reference-thread.34925/

Patrick has alot of other writeups on the STH main site.

Personally my preference is Lenovo P330 or M720q:

• Supports x8 PCIe 3.0 low profile card or a 2.5" SATA drive
• 1xM.2 for M720q; 2x M.2 for P330
• 2 DDR4 SODIMM slots
• 2x 10G USB3.2 ports
• 1xM.2 B key supports non-wifi cards also (e.g. Zigbee/zwave)
• Heatsink technically supports up to 65w i9-9900 (though you leave alot of performance on the table for sustained workloads), but most come with an i5-8500T

There’s also the HP T740 which also supports a PCIe card, but is a bit taller than 1U