A few other things to look at:
QD's can be disassembled. Take a large screwdriver and unscrew the top. It's good practice to clean your liquid QD's when you change kegs, but gas QD's can stand a little maintenance as well.
A shutoff valve on the gas line is a good idea (Or a manifold)
If you're using flare QD's instead of barbed QD's, you need to keep an eye on the fit between the swivel nut on the gas line and the QD. They can leak.
If you do use a manifold, you need to put a washer between the swivel nut and the manifold, as metal on metal will not seal reliably.
I could see a 5# tank running empty after 5 kegs, IF you force carb. That and too much "Gotta purge the oxygen out!". In other words, if your process involves filling an empty 5 gallon keg with Co2 then filling it with beer, you're wasting a LOT of gas. Put the beer in it first, then purge the headspace.
Just a few thoughts...