A tool (swep for gamemod) that shows you a selected entity (from Gui) in world (client side view, no collide/phys) that is 50% transparent and is green if place-able and red if not. The entity is on the ground where you have your mouse. A click spawns the entity if you have the money.
I would also like a tool (swep) that builds walls that snap to a grid in the world, but works as the other tool. This tool should spawn things like the hammer wall brush.
If you have ideas (code examples preferred) how i could make this or parts of this (not random links to unrelated tutorials) then please post them