Hi T,
To me an airbrush is one of the five basic things any serious modeller needs, along with glue, paint, filler and sanding material.
As for whether a brush can do everything an airbrush can, the simple answer is no. For a solid or splinter scheme you might get away with careful masking and sanding down your finish with 2000+ grid polishing paper, although the quality will rarely come close to an airbrush, but if there is any need for soft edges (e.g. luftwaffe mottle or soft edge british 2 tone) then no, the brush is totally outclassed. I've seen many attempts to pull this off with a brush but, to be brutally honest, they look awful.
You couldn't pull this finish off with a brush:
(not my work, but a great example of what an airbrush has over a normal brush, taken from
Mistel 1 by Floyd S. Werner Jr. (Hasegawa & ProModeler 1/48))
My advice would be to read up, look at
Airbrush,airbrushing lessons and how to articles. and get yourself a good airbrush and compressor (avoid cans of propellant, the compressor will pay for itself in a few months of usage compared to canned propellant). A low to mid range Badger or Iwata will work great (I have brushes from both and they are both good companies).
A wash is a thin mix of (usually oil based) paint which is run into the detail to make it more visible. For a good explanation and picture based tutorial, have a look at Mark Swan's website:
Weathering
All the best
Jamie
Airbrush,airbrushing lessons and how to articles.