You don't have to buy all the spells right as you level up, but I'd try to go back and backfill once you can afford it. Some really are junk though and can be skipped - things like True North or other super-narrow utility spells. Check out
this guide if you haven't already to help you prioritize what spells to get at each level.
For group buffs, you are right that the early levels you don't need to really use them. You could throw strengthen on the melee types if you're at full mana, as it will help, but the effect is so small they'll barely notice. You're buffs become valuable at level 16 when you get Breeze and Quickness. At that point your group members (and random passers-by) will be begging for your buffs.