The inside of the Streacom FC5 is equally spartan. There are mounting points for the motherboard, and a special bracket for an optical slimline drive. You can fit up to three 2.5-inch hard drives or SSDs, or two 2.5-inch drives and a 3.5-inch disk. The latter only works with mini-ITX boards. They can all be attached to the bottom plate with screws.
At first glance it looks as if a mini-ITX board would be perfect for this chassis, but that's not the case. The design with the heatpipes only leaves room for a few components and connectors from the CPU socket towards the right side. Mini-ITX boards are typically quite full with features and connectors, such ATX power plugs or SATA ports. Streacom has a somewhat outdated compatibility list on its site, which indicates that most mini-ITX boards won't fit in this case.
For the power supply you will need to use a so-called Pico-PSU, which is an adapter that you plug into the ATX connector on the motherboard. The external power supply is then connected to the Pico-PSU. The cable needs to be at least 40 cm. Streacom does not make these power supplies themselves, so you will have to find one yourself.