Depends a lot on your other modules and what you’re trying to do. Only have one voice module? Probably don’t need a mixer (unless you have multiple CVs you’re trying to mix, of course). Don’t have any modules that expect clocks (or repeating timed triggers)? Probably don’t need a clock. Not sure exactly what you mean by bus – a passive or buffered multiple? Buffered multiples are great for duplicating pitch CVs in your rack (i.e., multiple VCOs), otherwise passive mults are fine (or stackcables/splitters/etc)
If you’re just getting started, and you have a controller (pitch CVs & gate generator), VCO, VCA, and EG, you’re probably fine. Some modules combine some of those (e.g. Plaits is a combo VCO/VCA/EG) After that add an LFO so your patches have some life, and maybe a VCF depending on your VCO. But don’t get too much before you’ve actually started playing with the thing! Oh and try VCV Rack if you can – it’s a free eurorack emulator for your laptop, so you can build lots of patches and see what types of modules you end up using a lot