step 1: read The Conscientious Marine Aquarist by Robert Fenner
this is gonna answer 99% of your questions, including a lot you didn't know you needed to ask
I recommend a refractometer over a hydrometer, simply because they are far more accurate, and a lot quicker to get a reading with.
protein skimmers are a good idea...not required, but they make water quality better, and thus maintaining the tank is a little easier.
A 10gallon sump probably isn't worth it to setup...if it was a spare 20gallon, I'd say go for it.
Those rocks may not be live anymore...if they're dead, yo ucan scrub them down and re-use them as base rock.