HDPlex 200W Linear Power Supply

I’m considering getting a H3 V3 from Hdplex (i plan to use a mini rxt). Larry is out of stock on the power supply 400w combo and 200e dcatx as well. He suggested i use a external ac adpter plus picoPSU like solutions.

I don’t know anything about power supply’s. Any guidance on what I sould be looking at exactly - there are many variants ans don’t know what I should be using. Thanks