TD, painting the exterior isn't as bad as it looks. I just airbrushed the parts flat black while they were still on the sprue. I used Vallejo acrylics. But you can use whatever brand you choose. As far as the detail painting I did that just like I would on any other model. I used Tamiya Weathering Master kits to add a little depth to the flat black panels. If you aren't familiar with them there are 4 different kits, that I've seen, with three colors in each. Instead of the dry weathering powders they are like a paste that you can apply with the included foam-tipped brush. They give you greater control over just how much you apply and where you apply it. I used the Gun Metal paste to highlight the angles, edges and raised details on the panels much like you would drybrush paint to get the same effect. But the Gun Metal applied to the flat black is very subtle which is what I was looking for. When you look at the model the depth really jumps out without being overwhelming.
Unless you plan on making the roof removable you might not want to spend too much time on the interior. Once the model is complete the interior isn't very visible. And remember, there are a few left and right parts that are reversed in the instructions. They'll have to be switched around on the model.
One more suggestion. There is a book called, Batmobile: The Complete History, by Mark Cotta Vaz. It's like a coffee table book with plenty of color photos and illustrations throughout and it pays particular attention to the Tumbler. It's an excellent reference for your build. Mine wouldn't have looked as good without it.
Your Joker figure looks good. The face is awesome. I don't feel I'm very good at figure painting either. In fact I think painting in general is my one skill that can stand the most improvement.
Good luck on the Tumbler. Can't wait to see it.