I recently upgraded the CO2 system in my carbonation fridge with a 1:4 manifold with shut-offs and check valves and new tubing to the four QDs, all connected to an external tank & regulator through a stainless steel feed-through drilled through the back of the fridge.
With all the new or reconnected bits to test I bit the bullet and assembled the whole thing at the kitchen sink, then submerged everything except the tank and regulator (I checked all the regulator couplings and threaded bits using Star San spray). Kind of a pita, but it was reassuring to not see any bubbles at that point (especially at the manifold & valves).
I just had to disconnect the flares at the feed-through, take everything downstairs to the fridge, install the feed-through, screw the manifold to the fridge liner, then reconnect the manifold and regulator to their respective sides of the feed-through. And I sprayed those two connections with Star San just to make sure they were still good.
I've run that system for a couple of months, carbed up and held 8 kegs, and I'm still on the same 5 pound tank of gas.
Putting a CO2 system together isn't too big a deal to do if you use good components and assemble them as they're designed to be used. And if you have the opportunity to fit everything together for a leak test, it's that much easier to end up with a leak-free system...