I immediately noticed the problem with the brush.
It will be hard to understand, if you're new to this, but I'll try:
I noticed how a vertice is slightly offset, it's not in the right place. You should try moving the vertice with the VM Tool. Of course, you'll get a non-planar brush, but you can solve that by using Ctrl+F.
Ctrl+F is a command which splits a face into 2 separate ones. You basically select 2 vertices of a face, and you select 2 edges (which don't connect to each other), and then you press Ctrl+F.
If you don't understand maths very well, you might have a problem. Here's a resulting face after the process:
__________
|........./|
|......../.|
|......./..|
|....../...|
|...../....|...Ignore
|..../.....|....the
|.../......|...dots
|../.......|...please
|./........|
|/.........|
Simply said, it creates 2 triangles on a face.
So, by using this, you can fix the brushwork problem.
Now, as for the turret thing:
You just need to assign the rotating part a "func_tank" entity, which should shoot something at you. Of course, do not forget to add an origin brush.