Every thug you suggested seems really easy but I guess your a beginner do ill help you out..
I presume you have a basic understanding of construct2's event system.
You need a global variable called money.
Have a layout for your shop then for the purchasing event; something like this should do:
Money >= 1000 // if our current money is greater or equal to 1000 then we can buy
// then something to trigger the gun being active
// you could have a instance Boolean with a animation frame or something..
I'll make a capx tomorrow for you, I'm currently on my iPhone and don't have access to a pc right now.