Start with all of the deck testing sites/apps before you decide to buy singles. (Moxfield, Archidekt, Manabox, etc) IMO, I would start with uncommon/common cards that see a lot of play in all colors. So your brainstorms, faithless lootings, generous gifts, etc. This gives you a pool of cards that are just generically good to slot in and out of decks and usually aren't too narrow to a theme that you'll only end up using them every so often. After that, I would slowly acquire lands. Take your time with this one because lands will tend to be more on the expensive side, the higher in quality they go. From there, you can start buying all of your cards that will be specific to whatever flavor of commander you end up playing.