Are you looking for EOs exclusively or FOs as well?
My basic EO library would include lavender or lavandin, mint, spearmint, patchouli, folded citrus (lemon & orange), rosemary, lemongrass, litsea cubeba, eucalyptus, tea tree, clove bud, cinnamon bark, anise, and cedar. All of these are pretty reasonably priced except the lavender and the patchouli. You could make a lot of different blends with these options. If you wanted to splurge, I'd add cardamom, fresh ginger (the dried variety is pretty pungent), nutmeg, and some other conifers - spruces or cypresses.
If you're just starting out, the prices at Camden Grey are very reasonable. I also buy from New Directions Aromatics and Liberty Natural.