Here it is in a nut shell,you need proper ventilation,you can get by without it but why,when you start out with proper ventilation you may never know what it's doing for you but when you start out without it,then start having problems with temps,humidity,mold,smell ect,it's then you realize how badly you need good ventilation.
It's much easier to put in vents in a new grow room before it comes to life than it is after its a cramped forest of goodies.
To top off that big speach i just gave your over thinking the whole duct work deal,ventilation can be extremely simple,all you need to remember is fresh air & use a common sense approach,start out by figuring out how to get rid of the hot stale air,heat rises so the hottest air will be near the ceiling,find a way to run an exhaust fan from the ceiling out of the room,this can be as simple as cutting a round hole in the wall & sliding a round inline exhaust fan into the hole then putting some caulking around the fan to seal the hole & hold the fan in place,then add a standard 3 prong cord to the fan wires & plug it in.
Now that you've got the issue of removing heat under control you need to think about bringing in fresh cool air,you have 3 choices,you can add a passive air intake which is nothing more than a hole in the wall or you can add a powered intake which once again a simple inline fan is used,with a passive intake if your exhaust fan has high enough cfm's to it the fan will pull enough air from the room where the passive intake will have fresh air pulled through it,if the exhaust fan is not of a high enough cfm rating then a powered intake should be used.
The best solution & the one i chose was the 3rd choice,i used 2 powered exhause fans at ceiling level to remove hot stale air,then i cut 2 passive intakes at floor level to allow the exhaust fans to suck in fresh cool air.
Post detailed pics or drawings of your room & i'll show you how.