I'm not sure I have any advice to offer on that, I mean if I had 120 polygons to spend on a tank it'd look just like that, you've managed to keep the polygons under control and it's a neat model.
As you say, it's viewed from a distance, lower poly meshes look better than high poly from a distance because the details look terrible, especially when you get to that scale where the details are often just 1 pixel flickering about and looking terrible as you move, a low poly mesh is much clearer.
Actually in Star Wars, they had several models of the milenium falcon, they'd use different models of different detail depending on the distance to the camera - it's a very similar problem.
It would be a shame if you stopped, because your on a roll - use that tank as a basis for all your tanks, and try to get all your tanks done, even if you don't draw their textures just yet. It's so quick and easy to take a decent model and spawn variations of it, you'd be daft not to capitalise on your new found skills.
Actual drawing of the textures could use a little work. There's a few methods to make tanks look cool really quickly, like using plates with rivets, camo, stuff like that. By doing a whole batch of them at once you can save a ton of work and end up with tanks that all match in style - giving your project an extra element of continuity.
Van-B
Put away, those fiery biscuits!
