nice work herr_klaus, the tank is looking good - maybe some deeper shadows are needed around the wheels in the track as they are looking very flat. Also putting it in an environment with the relevant lighting might show it off better.
I'm on with a head at the minute too and know how difficult it can be. I think you might have a few too many lines for the simple shape it currently is at. Dont know whether the loop that attaches at the top outside of the nostril will help if you plan on animating it but I'd probably try and simplify it a little.
The general nose shape is looking pretty good