How are they basic to homebrewing? I gotta to agree with janx here...unless you enjoy getting data to store or enter contests or have some specific reason for wanting SG then it is very much not a basic need. I've been brewing for about 6 months now all-grain and have probably made 20+ 10...
I'm not quite sure what kind of keg you mean but the ~15 gallon (~50L) 1/2 barrel kegs are ideal for brewing. They are about 16" in diameter if i remember. Just be sure to get cookers that are big and beefy enough to hold them securly. We eventually found some really perfect one for those...
I dunno, so far in my brewing experience i've found dispensing to be kind of black magic. My system is soda kegs in a fridge with line for about 6 or so feet to my taps. The two regular taps pour pretty foamy usually...which we haven't been to motivated to look into but i've found a lower...