I apply the gesso with a brush as primer. I'm starting to use Vallejo Surface primer (#74.600) now as well. It works great airbrushed and hand brushed.
For top coat, I used to use Delta Ceramcoat Matte Interior Varnish. I had an older bottle dated 2004 and it seems they changed the formula a little bit and its not quite as matte as it used to be. Also there is an exterior/interior type that is more glossy than just the interior type.
Vallejo also makes a nice matte top coat but its also a hit or miss if the bottle will be truly matte. I've got one really matte bottle, and the other is much more glossy.
Krylon Matte finish is really great but it is a spray and you need a lot of ventilation. The other ones were brush on.
Lately though I have not been top coating. I feel it's not really necessary unless you have interchangeable parts and will be handling the painted kit a lot.