Yes, Mantaroth is a beast and a must-have. I've won a few lower mana battles with it just solo. lol I'm kind of doing the same thing you are except my goal is just to bring my whole deck to gold level, "bad" cards and all. I play in diamond mostly and it always amazes me how often cards that don't seem that valuable manage to steal the show sometimes. Of course, I would probably be better served learning how to play a smaller deck because I end up costing myself on a regular basis by trying to weed through all the different options.

But, it's fun, which is the important thing. Good look on building your deck. Not to throw cold water on your plans but....you quoted a level 3 Mantaroth for over $100. That's actually only a level 2. Level 3 is over $200. Unfortunately (or fortunately, depending upon your perspective), since it's a promo card, it's out of print. I don't expect that one to come down too much in price over time. In fact, I think it will just go up. At least until another card comes along that can match it.