You will be fine with a 9.9. Don't get a long shaft unless your transom requires it, the less drag you have the better she will run. However, you could buy a 15 hp motor and either switch the hood to a 9.9 or paint it and re sticker it to a 9.9. Almost every manufacturer the chasis, powerhead, bore/stroke, pistons, weight, cosmetic components, props, and housings are the same for 9.9hp and 15hp outboards. What separates the two types of motors lie within the fuel systems, manifolds, cam shafts, and exhaust tubes. I doubt those would get checked unless you were doing something crazy with your boat.
Some may suggest to you to buy a 9.9 and put a 15 carb on it. Some of the older outboards you might have been able to get higher performance by just swapping the carburetor or jets. For the most part this is not enough. Outboard manufacturers make outboard engines tuned to a particular horsepower. If you risk swapping components with the expectations of gaining more speed you may be pushing your engine too hard. This can be a result of neglecting the exhaust or intake manifolds when converting your outboard. You have to account for all the details or you will do serious damage to your outboard.