I have the Trumpeter kit as well, but it didn't look that bad when I first saw it. The second one that you have, with the dozer blade, is i nfact the same kit, with a few extra sprues for the additional parts.
As to your undercarriage. I would say one of the key factors in wheeled AFVs are the tires. It's very important to portray correct weathering and dusting, as it just looks great. Notice how in the grooves of your jeep, there is no dust. You should attempt to paint the tire, drybrush in a dark gray, then dust the c%$# out of it. I prefer the use of pastel chalks for this. Really work it in there, then dust most of it off. The dust should remain in the grooves, treads, and a little bit on the sidewalls. After that, drybrush again with a lighter gray (but still fairly dark) over the circumference of the tires, so that the treads remain dusted, while the contact portions remain fairly clean.