If you don't buy an Optiplex with the M.2 SSD option, Dell doesn't solder the slot onto the board so you can never install one after the fact. : r/assholedesign
Amazon.com: Fast DELL Optiplex 3020 Micro Desktop Computer Ultra Small Tiny PC (Intel Quad Core i5-4590T, 8GB Ram, 256GB SSD, WIFI, HDMI) Windows 10 Pro (Renewed) : Electronics
Dell Optiplex 3020 Micro: A business PC that's small enough to fit anywhere [Review] | BetaNews
Support for OptiPlex 3020M | Documentation | Dell US
Amazon.com: Dell Optiplex 3020M Micro Desktop Business Mini Tower PC (Intel Quad Core i5-4590T, 8GB Ram, 500GB SSHD, WIFI, Bluetooth, USB 3.0) Win 10 Pro (Renewed) : Electronics