Yoshi,
Welcome to the forums! Nice choice on the BD795M, that’s a really solid little board for a home lab build. Since you’re not planning on using a rack, I think the Lian Li A3 is actually a good pick for what you’re putting together. Your Cooler Master 850W is a standard ATX power supply and the A3 looks to have a relocatable PSU bracket that supports full-size units, so I believe you will be covered there. Just be prepared for cable management to get a little tight once the NIC and storage are in place.
The Noctua NH-L9i-17xx is low-profile so clearance won’t be an issue, and the specs of the A3 show it supports up to 165 mm tower coolers if you ever decide to step up later. For the Intel X550-T2, you’ll be fine too. It’s a dual-slot card but the A3 has the room for PCIe devices. I also like the mesh front panel design as it will give you decent airflow so things stay cool.
If you wanted to look at other options, there are cases like the Fractal Pop Mini Air, Cooler Master NR400, or Jonsbo D31. These all support ATX PSUs and have good airflow, but honestly the A3 is one of the most space-efficient and clean-looking mATX cases right now. Given the parts you’ve listed, I don’t see any issues with compatibility. If you like the design, I’d say go with that one.
Brandon