I suggest doubling the spawn range or make some kind of warning for spawning ships. You should also make the money (pips?) less colorful, they clog up the screen and you cant see the enemies. And last but not least, make the health price not so drastic.
Flamethrower is stupidly broken. Mortar sucks...Flamethrower for time and killing medals. homing missiles for health medal(because going up close gets you shot) game beaten.