best exhuast set up

[Berlin]

What do you guys think is the best header setup and the diameter of the down pipe? I have at my disposal for a limited period of time a hydraulic pipe bender, welding skills (lasting), and SS tube stock. but what would be the best diameter to go with, should i try to match it to the stock 4-2-1? (which is quite skinny)... and what cat-back to go with? anyone with experience making customs exhausts--give some tips please, is this doable?. thanks in advance (: