I've been riding motorcycles for the better part of 25 years now. Dirt bikes, Enduros, and Street bikes. Everything from a 50cc Honda, to 1500cc monsters able to easily exceed the national speed limit in first gear
Personally, I'd recommend the biggest CC bike your dad is willing to pay for......for several reasons.
(1) Smaller CC bikes, such as a 250CC Ninja, are generally very light bikes, with an incredible weight to horsepower ratio. While this sounds neat, it is to a certain point. Smaller light bikes with high horsepower tend to be a bit more jittery than their larger breathren in my opinion....This is especially true if the weight of the rider isn't very substantial. Meaning, that I feel they seem to get away from you more easily. Don't confuse that statement though with raw horsepower or top-end speed. Obviously, the larger the engine, the more torque its likely to generate, and the faster it can go. Because of this, they too can get away from you very easily. I just feel that the fine line between total control, and out of control, is a little wider with a bigger bike.
(2) Getting too small of a bike, especially for a beginner, could lead to over-confidence in your riding ability in a short period of time. Don't get me wrong, confidence while riding a bike is a good thing. Over-confidence is always bad. You'll be tempted to try things that your abilities might not be up to performing. Especially if you are trying to "keep up" with your friends who have more riding experience than you do. Make no mistake, no matter what size bike you get, the minute you fail to respect it and what you are doing, you're inviting potentially disaterous problems!!
(3) Getting a larger bike has some advantages over the smaller ones as well. Resell value is generally higher, you won't outgrow it as quickly, and it will more often than not have a more smooth powerband and greater torque at lower RPM's.
I'm not necessarily against used bikes at all as a way to save some money, both on the initial purchase, and on insurance. You just have to be careful to know what you are getting and its condition/treatment history. If you're not a mechanic, and don't know the history of the bike, its difficult to evaluate. Often, in order to be sure what you are getting in a used bike is a sound purchase, its best to buy from a bike dealer who has inspected the bike and made sure that it is worthy of being on their sales floor.
I also have mixed emotions about your choice of motorcycle. I'm not 100% sure that a Ninja type sport bike is the best choice for your first motorcycle, but can't say that I have a definite alternative suggestion either. It would be like getting a new sports car as your first automobile. I understand the attractiveness of the model though.
Will be interesting to see what others think and advise you.