it depends on what type of tank you want to go with. If its fish only, then you could go a really really long time w/out water changes. If its a soft coral tank, you can still go quite a long time w/out water changes since they dont really use much of anything in my experience. I've gone a month before w/out doing water changes on my 40 breeder softy tank, which is loaded w/softies/leathers and a few lps and a couple sps and i dont even run a skimmer. As long as the levels stay good, i see no point in doing a water change.
But a general guideline for your 12 would be to do like 3g every 2 weeks or something like that.
As for the lighting on your 55...are you gonna try doing sps and stuff that needs lots of light? If so, i'd think that 2 separate 175w MH bulbs would do the trick. As for how to make them....look at the DIY forum on here. They'll be able to help you make them pretty easily i'd think.