To pass code, a subpanel needs the neutral and ground buses separated. How you do that, depends on the panel. Many offer add-on bus bar extensions.
50A might be pushing it if this includes a bathroom(s) and maybe a computer room or some TV's. Probably okay, but maybe not. If it truly is mostly just some lighting, it should be fine. But, some people end up with some big current hogs, like maybe a window a/c unit or two, a small refrigerator, etc. A laser printer can pull a fair amount when the heater element turns on. It all adds up.