Never seen an 18 gallon keg ? Your yonger than me ! All we used to have at parties once , the spear is not in the keg , you used to fit it at the time.
And if you did'nt know what you were doing look out , may black eyes and broken teeth from that one.
The jerrys are food grade and about $20 from K-Mart etc 'Willow" brand
The air space in the jerrys , I give it a shot of CO2 from my bottle , it's heavier that air so sits on the top of the brew , never had a problem anyway
Cheers Batz