Ideally you want 1lb rock per gallon, though you can start with less and add more later if cost is an issue. The cost of that can really vary depending on where you get it, but it is not cheap. Could easily spend $100-150 on it. Protein
skimmer would be a very good idea, particularly if you want to keep inverts and/or corals. Cost $100-125 minimum. 2 powerheads...$20 each. I assume you already have a heater since this is an existing tank...$15-20 for a new one if not.
hydrometer or
refractometer for measuring/maintaining your tank's
salinity...hydrometers are pretty cheap, but not as reliable as refractometers, which you can get for $40-50. Salt mix...$20-25 per bag, which typically mixes 50 gallons. You can get the salt cheaper online, but by the time you pay shipping you are right back to $20-25.