Just about anything you find on Airfield models is going to be great advice. It's an outstanding source for modeling information.

As for an easy and quick to use balsa filler I just use Elmer's wood filler. It's cheap and does a great job in filling, and it's easy to sand. Another great filler is lightweight spackling material.

Micro-balloons are a material that you can put in epoxy that will give the epoxy more filling and sanding capacity. Mixing them with epoxy makes for a great material to use in filling in gaps, seams, and holes in your model. As with a lot of things that we use in this hobby there are household substitutes that you can use in place name brand items. Instead of buying micro-balloons at the LHS you can use corn starch.

Hope this helps

Ken