I think you have a great attitude and it will serve you well. Nothing wrong with admitting you are looking for an adrenalin rush. Knowing that and keeping it under control is what will keep you safe and riding for many years. Starting out on smaller bikes is the way to go as it will help you learn how to ride. The fun part is taking corners well and you can take those corners faster if you master the proper technique. Simply accelerating/braking from corner to corner will teach you nothing.
Get yourself a copy of Proficient Motorcycling by David Hough. That is the bible when it comes to learning how to survive on the streets. There is so much more to learn than simply mastering the bike. And that is part of the fun.
Good luck finding the bike you want but I would not rule out the Ninja. They are very common but for a good reason, they are great bikes (at least according to everyone who has ever ridden one). Having a great handling bike that you can learn on is no bad thing. The fact it is the bike every beginner wants is no bad thing either come the time you want to move on. Selling on your 50cc might me more troublesome.