Nerf guns. Bondo. Miliput. Metal tape (like they use for duct work) Spray paints (I prefer Krylon ultra flat colors) and blue painters tape for masking.
Check out Johnsonarms nerf prop gun thread here for inspiration on mass effect mods for nerf guns too. He's done quite a few of them now:
http://www.therpf.com/f9/nerf-gun-props-yes-106425/
Modding toy guns and the like is a great way to hone scratch building skills, BTW.
If you hit any specific road blocks along the way, post up photos and there is always somebody around to jump in and offer suggestions.