basically i'm looking for a way for a brush model to have a texture, buyable, AND the zombies can move through it in case one of the players go down, or in solo play, the zombies can get from one area to another.

help would be very much appreciated

non colliding and a player clip

And just incase you didn't understand that. Select the brush model, right click on it in the 2d view and select make non-coliding. Then select it, duplicate it (press space bar) and apply a player clip texture to the new brush and position it over the non colliding brush.

Like wise to make a wall that players can go through but not zombies, make it non colliding and put a monster clip over it (I think, correct me if I'm wrong somebody

i kind of failed and made my player clip non collide, how do i make it collide again?

remake it

Alternatively you can right click it and click 'make structural' or 'make detail'

I've always "made detail" the bits which are detail, mainly so I can filter them out if I need... but what actually specifically does it do interms of bsp/drawing/whatever ?