Decide the profile you want to type on (for me it's SA). Next choose the color scheme you are looking for. Lastly look into materials and see if you want ABS, PBT, etc. Now you've narrowed it down to 1-3 sets. Then it's up to you to decide if you want a used set (best option for more unique sets and it's available today), off the rack, or something off the shelf. If you want to go group buy you could be waiting over a year to finish your keyboard.
I strongly recommend looking on the used market. These are solid pieces of plastic, they won't be broken, damaged, unfixable. A quick rinse to clean them and they may as well be new. There are also sets that are now out of production and may never get remade. SA Camping is a good example. You can get Camping in GMK, but not SA. SA Amazing Chocolatier is not available since it's first release on Drop. There are a lot of older sets that would/could make your board pop and stand out from the crowd.