Exhaust looks good Corbin. Definitely tape up all the joints, purge from the low point and bleed out the high point. It should not take a bottle (depends on your bottle size I guess) to purge it out. The average calculation I use for sanitary work is at 25 CFH, it takes 1 minute per foot of 4" tubing. It goes without saying that tight fitups are always nice to have, as any gap bleeds the argon pressure out and requires a higher purge flow.

And on your DP/mid pipe assembly, is that a sanitary reducing fitting? :)