1. perfectly a-ok - brass is just more $$
2. my answer: short & fat (within reason). that also assumes you have a tube or straw taking water from the bottom, not top. I think too tall & skinny doesn't allow for enough water movement & turbulence, which is what breaks up the bubbles
3. not really - other than circumference & dimensions - see #2
4. I'm pretty sure a DIY will be pretty quiet. The biggest noise is the impeller blades on commerical models like the Ista Max Mix. DIY models, no moving parts, no noise.
5 & 6. The more stuff you put in, the more maintence you're asking for. Even if it's inline after the filter, pot scrubbers & diffusers are eventually gonna clog up.
...just my $0.02