That series of desktops use a proprietary small form factor power supply from HP that is setup to match the unusual wiring setup of the motherboard. HP does have up to a 500 W PSU for it with some models, but I understand they are not so easy to find. The HP store has them on back order and I dont know if they will ever be available.
Your system already has one HDD already mounted. There is no room for another, but there is an M.2 slot available that you can mount an M.2 NVMe SSD. OItherwise you will have to leave your other external drive as an external drive and connect it via USB.