Here is a list of items and how many of each rarity of mod is needed for each higher level assembly
Advanced (6 can be built with only basic mods)
chainsaw 3 normal
rocket launcher 3 normal
pistol 2 normal, 1 exotic
chaingun 3 normal
pistol 2 normal, 1 exotic
chaingun 2 normal, 1 exotic
BFG 9000 2 normal, 1 unique
any boots 3 normal
any armor 2 normal, 1 unique
any armor 2 normal, 1 unique
any boots 2 normal, 1 unique
any shotgun 2 normal, 1 unique
plasma rifle 3 normal
double shotgun 3 normal
Master (2 can be built with only basic mods)
any ranged non-shotgun, non-BFG 3 normal, 1 unique
any 10mm-based weapon 3 normal, 1 unique
any armor 2 normal, 2 unique
BFG 9000 1 normal, 2 exotic, 1 unique
chainsaw 3 normal, 1 unique
any boots 4 normal
any armor 4 normal