1. yep you got that right - pfeeew!

2. ok

3. well you can't buy something if you don't have the money can you?

so you just need to check

if value_of_cool_gun is equal or smaller to Total_amount_of_currency - if not then just sent a message "go get a job, space dude"

4. when you buy a bomb you set a variable on, let's say Got_bomb = 1

then when the player press the key for the bomb you just check if Got_bomb == 1

All the questions here are of pure logic and don't depend on Flash features.