Hi!
Modge Podge is a good sealer, a lot of people use Modge Podge matte (I find gloss modge podge to be tacky when it dries). There is also a spray sealant called Testor's Dullcote that is amazing.
As for airbrushing, it is a pretty big investment, even if you go the less expensive route. You can sometimes find a very basic airbrush for maybe $20, but it is harder to use with less consistent results. Airbrushing has a quite a learning curve, so if at all possible, it's better to go for the mid to high range airbrushes to save big headaches ($60-$100+). The air source is another issue. While they do sell canned air for maybe $15 a can, it will hardly last through one or two ponies and starts to add up fast. A compressor is really worth the money, but it's more on an investment than the airbrush!
The good news is a lot of artist get very smooth results with a paintbrush, practice, and many thin layers. It takes a while longer, but it's a heck of a lot more affordable! So there are several options, it really just depends on what you feel comfortable spending. Good luck, can't wait to see what you create! Hope this helps a little!