90-100 minutes is waaaay too long to bring it to a boil. It won't hurt anything, but it will drive you mad. Could you just boil 3 gallons and then top off with water at the end? I assume you were going to top off anyways.
Also, see if you can straddle two burners with your pot and turn them both on high. That's the only way my stove top will get my water boiling.
If you can afford the turkey fryer, get it. You won't regret it one bit.