First of all Welcome to Aquarium Advice!
basics - tank, stand, heater, 2 powerheads, filter, light, liquid test kits for pH, ammonia, nitrite, and nitrate. You'll also want a refractometer to check the salinity.
Different people like different brands and have different budgets so you'll need to keep this in mind. This is what we have for equipment in our 38 gallon, all would work fine in a 50. AZoo (? I think that's the brand name) Titanium heater, the powerheads are 1 Petco store brand, and 1 AquaTech cheapie from Wal Mart - I personally like the Petco brand better than this one as it has only 1 suction cup that isn't strong enough for the weight of the powerhead , We have 2 filters running: 1 Whisper 60 hang on the back, that one is just increased water movement and biological filter (we have a large colony of pods living there as well), the other is a Fluval 204 canister set up with LR, ChemiPure Elite, and purigen For lighting you can go with standard hood with fluorescent T8 bulbs if not doing corals - if you decide to go with corals down the road you can upgrade the light. We use API liquid test kits. We purchased the tank with a stand, lighted hood, heater, and filter, and misc. other accessories from Craigslist for $40.....so you might want to consider looking there, just make sure the tank doesn't have a leak - out of 3 tanks from Craigslist one had a leak......it was still a really good deal, though with all of the extras that it came with. We switched out the light for a Coralife High output T5 4 bulb fixture with lunar lights and replaced the heater with the titanium one. We purchased a nice refractometer from ebay for around $30. I'm sure others will chime in with name brand suggestions for you as well. You may not need the 2 powerheads if it's just a FOWLR aquarium, water flow is never a bad idea however to keep debris off the rocks. Doctors Foster and Smith online is a good place to begin looking for some equipment and they are knowledgeable and helpful, sometimes their prices are better than what I have found locally.
You'll also need substrate, sea salt to mix your water (We use Reef Crystals), water conditioner if you have chlorine in your tap water, live rock, and probably base rock. You'll also need an ammonia source for cycling - I reccommend ACE brand Pure Ammonia.
Welcome to Aquarium Advice and good luck with your upcoming saltwater aquarium.
The big thing with saltwater is to be patient, especially through the cycling process. I would suggest that you look up info on fishless cycling, if you're not familliar with cycling a tank properly. We were new to saltwater last year found this forum helpful and encouraging. Here is a link that you can read through
http://www.aquariumadvice.com/forums/f14/classroom-setting-up-10-gal-qt-this-afternoon-173481.html
This link is the beginning of our journey with saltwater.
