Assassination Faction

[deathmotto]

I want to make a faction like the brotherhood in oblivion (i think thats what it was called) where you get assassination contracts. I would make it myself but i m getting used to G.E.C.K and i seriously have no clue how to make new models for this; such as a rifle which gets upgraded as you get more contracts. (like more modifications or something such)

 

I would appreciate it if someone made something like this, or at least taught me how to create models for new weapons and armor.