[sushiboat]

My idea wouldn't be completely "instant" in the sense that the research you buy still wouldn't be available until next turn -- just like recruited/summoned units. The issue of delaying commitment to a research path until you know more about the enemy is a concern. However, if you are hoarding RP, that also means those spells aren't available to you in the meantime. Perhaps there should be a constraint so that you can't purchase more than one level in a school per turn. Another possible constraint is that you can only spend a certain number of RP per turn, based on last turn's RP "income" plus, say, 20%.

Edit: There could also be more than one-turn waits for purchased research levels. For example, levels 1 and 2 would be available next turn when purchased, L3 and L4 would be available two turns after purchase, etc.