For the extra 40-60 dollars, go for atleast a 30 gallon tank. That size tank is very hard to maintain. Check local reef forums for cheap equipment. May i suggest this setup for 250. All good except for some aiptasia.
http://www.azfrag.com/forum/viewtopic.php?t=768 Very nice deal imo.
Powerheads are helpful but not necessary as long ad you have enough flow elsewhere.
Salt is different for any person you ask. I use instant ocean and reef crystals on my reef and they do fine.
Aragonite is the material they make the sand out of. It buffers the pH ( i think). It keeps waste from getting trapped and is full of places for bacteria. You can have the sand bed as deep or as thin as you want. If you do however, want to lower nitrate with a deep sand bed, 6" in minimum. Deep sand beds however are not necessary.
API does has a master kit. Test for ammonia, Nitrite and once cycled, nitrate. Also check pH. Calcium should be checked when you get stony corals.
Skip the canister and go for a sump/refugium and/or a protein skimmer and live rock. A protein skimmer removes excess food from the water and helps keep nitrate lower by leaving less food to decay.