Mac Studio or macbook pro if you want to run the larger models. Otherwise just a gaming pc with an rtx 4090 or a used rtx 3090 if you want something cheaper. A used dual 3090 can also be a good deal, but that is more in the build it yourself category than off the shelf.
I went the 4090 route myself recently, and I feel like all should be warned - memory is a major bottleneck. For a lot of tasks, folks may get more mileage out of multiple 3090s if they can get them set up to run parallel.
Still waiting on being able to afford the next 4090 + egpu case et al. There are a lot of things this rig struggles with running OOM, even on inference with some of the more recent SD models.
Sorry if this is a silly question - I was never a Mac user, but I quick googled Mac Studio and it seems it's just the computer. Can I plug it to any monitor / use any keyboard and mouse, or do I need to use everything from Apple with it?
You can, but with some caveats. Not all screen resolutions work well with MacOS, though using BetterDisplay it will still usually work. If you want touch id, it's better to get the Magic Keyboard with touch id.
Any monitor and keyboard will work, however Apple keyboards have a couple extra keys not present on Windows keyboards so require some key remapping to allow access to all typical shortcut key combinations.