Well, the first one sounds like a
DESTROYER TANK HUNTER
http://www.forgeworld.co.uk/destroyer.htm
The second one is not legal really at all. If you and your friends agree to use whatever rules for the tank then thats ok. But your cant use it for any competitions.
The third is not really legal. The closet you can get to that is a Cadian pattern sentinal. (closed compartment so its not opened topped).