Start on an cheap model or the most expensive you have..Rule one in weathering is observe the prototype before you start. Rule 2 and 3 are the same. I search the net for pictures or whatever I can lay my hands on to study the prototype before I start on the model. As long as you know where the dirt goes you be fine. Watch the masters at work, search the youtube for Michael Rinaldi, Mig Jiminez, Adam Wilder, they are a couple of artists out of the extraordinary and are willing to share a lot of techniques. I'm personally not a big fan of powders only. They are IMO a nice addition to the layers of paint used. That's most of the time the big trick in good weathering that you work in layers, many layers. As Michael Rinaldi once stated: You washed your car and it's nice and shiny on the driveway. After a day the first layer of dust and dirt appear. When you wash your car once a month like me there are 30 daily layers of dirt on it.
Well doin demonstrations for several brands of paint I don't advice one brand. What I do advice at the moment that you as a starter work with the most forgiving medium. That are for me oil paints. Proper laid down on a piece of cardboard, used in thin layers the are a magnificent medium to work with. And they give you a long time to work them over. Not satisfied with the result, take some cotton swaps, odorless thinner and of it goes..