Welcome to the hobby. I got a Shapeoko 4 last year and it is a great tool.

1) some sort of dust collection solution. I use a Festool dust extractor which is very expensive but also very quiet. Others use a shop vac. But you need something.

2) workholding. The Carbide 3D get a grip kit, some masking tape + super glue (C3D sells a kit), and double sided tape are all I've needed so far.

3) bits will vary based on what you want to make. I'd target having enough variety to get you started while you learn the non-trivial software side of things. This could be as simple as a 1/4" spiral, a 90 deg V bit, and maybe a smaller spiral like a 1/8.

4) some useful tools: a dial indicator (to tram your router), a digital caliper, some sort of sharp knife for various cleanup and other delicate cutting needs, and a flush cut saw to trim tabs after you cut out a part.