As for your original question, part of it depends on the size of tank you are planning on, but from what I've read, for a smaller tank, the bare minimum would be along the lines of:
1 lb. of live rock per gallon
Sand for substrate
Enough powerheads to provide a flow rate of about 10x the size of tank (that is, about 300 gallons per hour for a 30 gallon tank)
Hydrometer to test salinity
General test kit for ammonia, etc.
In the future, you would want to consider:
Upgraded lighting to keep corals, etc.
Skimmer (some seem to consider this a necessity; others optional until corals)
unit (again, some argue this is a necessity; others not)
Take this response with a grain of salt as I am at the reading stage myself and am planning on setting up a tank, but have not done so yet.