It isn't a very complex plan -- it just required that he play two roles; this gave him two avenues of success (if resistance to Voldemort is strong, take over as the hero -- if resistance to Voldemort is too weak take over as Voldemort). And it fits in perfectly with the plan that he has already suggested to Harry Potter (find an actor to play Voldemort, have him cast Avada Kedavra at you, block it with Patronus, be hero), so it's perfectly consistent with Quirrel's way of thinking.

When we talk about too complex plans, we talk about plans with multiple points of possible failure. You call this complicated because it had multiple points of possible success.