if you get the 3k compressor you've pretty much limited yourself to filling the gun directly from the compressor. that could get tedious after a while too. also, tom kaye warns that the auto shutoff isn't precise anyway. so you still need to keep an eye on it even if you get the 3k compressor.
if you get the 4500 then you can fill scbas and fill the gun directly if you need to for some reason (again, as long as you keep an eye on it...)
tom did say the 3k compressor may be faster than the 4.5.
scba seems the only way to go. I got a 44 cu ft on ebay pretty cheap. joe brancato sells used ones for a good price too. the other "low cost" option is a pigmee from air hog for $275. you can get about a dozen fills from that and it should fill from the compressor pretty quickly.