Looking good savage! What I know about these vehicles could be scribed on a PE wingnut. I am finishing a AER T38 now as well and from that I can see somethings that might help. The horn is not hollowed out. The engine grill could do with a good dark wash. I think a wash with some burnt umber and some drybrushing with a lighter shade of the green would lift the detail up better. Another thing I do is to spray the running gear and hull bottom with the base shade and some dark earth added as it would be stained over time. When all this has dried do some spot washes of burnt sienna to simulate rust around bolts and raised details. Your rusting between hull plates is in the right direction. The wheels had rubber so these should be black.
Click on
this link and look at the pictures of a restored T26 which might help you.
Overall its a nice build and with the addition of track and decals(?) will add some colour and life to the model. Points above are only if you would like to take the weathering further!