Woah.... You should really make this a proffessional app. Amazing. Not to put down Kongregate, but is it really worthy of this program?!
(the only thing I have of note to dislike is the decoration and presentation. I know the focus of this was the programming but this can be another phase 2 think about)
I reckon the plane image shouldn't change by buying a model upgrade, it should change along with basic physical upgrades such as lightweight, aerodynamic, rudder control, engine (and maybe some others not currently existent), with fuel having its own upgrade. I've noticed that when I play I've fully upgraded long before the last area, so I think a few more upgrades shouldn't hurt. This is just a suggestion and sounds quite tedious to make (I don't know anything about game designing), but this is the sort of thing a lot of players like to see.
The only real flaw I see is that the monsters level with you, meaning that the only way to improve over them is through skills and equipment. However, once you've learned all the skills, levelling becomes NEGATIVE for you because the enemies slowly just get way to hard to kill. I don't oppose levelling the monsters, just reducing the acceleration of their power.
I really think this is an abuse of women, using them as tools (drawn, painted or real) to attract people to things. Why can't we admire women for their skill, manner and ways rather than their looks?