Q1 : it is a bad idea to use "wait", if you click again while your magazine is empty and the gun is reloading, the actions will be executed as much time as you click.
Set a variable named "reloadTimeLeft" to the number of seconds your gun needs to reload. Then subtract "dt" from it every frame. Don't allow the gun to shoot while this variable is greater than zero.
Q2 : you obviously have a problem with the way Construct 2 performs string concatenation, see the manual entry for strings, also, I believe there's a tutorial on advanced expressions that talks about it.
Q3 : IMHO leave the shop for later when you get a bit more experienced, try to tackle the basics first.