Alright, I'm going to try to answer all of your questions but let me know if I miss one!
First off, here's some reading for you:
Cycling Info: http://www.paw-talk.net/forums/f77/c...ium-43789.html
Goldfish Info: http://www.wetwebmedia.com/fwsubwebi...fshsystems.htm
If the big tank holds water and is indeed about 40 gallons, I'd consider using it. Before I even read that you had a large tank I was actually going to recommend getting a 40-gallon instead of a 30-gallon. The long shape (giving you good surface area) and extra volume makes a huge difference.
2-3 fancy (this is important, avoid "common" goldfish as they get even bigger!) goldfish would be happy in a 40-gallon provided it's properly set-up and maintained. I would stick with just the one fish for a while, though, until you're used to maintaining the tank and everything is stable. I wouldn't get any other fish, goldfish are best housed with their own kind. You can, however, get some nice live plants to make things more interesting. Goldfish do enjoy eating and digging up plants, though, so don't get expensive ones! Java ferns, hornwort, and duckweed often hold up and/or grow fast enough that the fish can't completely demolish them. Anacharis, water sprite, and java moss are others you can try. All of the plants I've mentioned tolerate cooler water temperatures (most actually prefer cool water!) and grow with little light and no supplementation.
You'll want the strongest filter you can afford. Goldfish are some of the messiest fish you can get, haha. I would start off with a HOB (hang-on-back) filter that's rated for a tank much larger than yours, or go with two that are only slightly over-rated (like two filters made for tanks over 55 gallons or something). Thinking about it, two filters might be good in case one stops working. Redundancy is always good in an aquarium. Once you get the hang of things, consider upgrading to a good canister filter. Canister filters are nice in that besides being powerful they also have a lot of room for filter media. They can also be stored under the cabinet, out of sight. I like Eheim models, personally. I would recommend one now but I think it'll be easier to get the set up going if things are kept simpler
Oh, and as for HOB brands, a Whisper or AquaClear should work. Maintenance would consist of cleaning out the sponge (or similar material) with water as necessary (probably once or twice a week for goldfish), cleaning out the filter itself (maybe a few times a month, depends on how clogged things get), and replacing any filter media used up as per the instructions (you probably won't need anything special but some people use specialized media to help control ammonia or activated carbon to polish the water).
I was really confused when I read that part about marine salt. I think you misread another post of mine where I mentioned using marine salt for a molly. Goldfish are freshwater and don't need any salt. Freshwater aquarium salt is good to have around (it can help with some infections and parasites) but it's not something you want to add for no reason.
The only other thing I can think of is to give you a "shopping list" of stuff you'd need. This might not be complete, I worked all day and my brain is fried at this point of the evening!
-Aquarium (20 gallons minimum for your current fish, 40+ gallons if you want more goldfish, which I'd recommend anyways since they're social)
-Two over-rated HOB filters OR a single extremely over-rated HOB filter OR a good canister filter
-Freshwater aquarium test kit (you'll at least want tests for ammonia, nitrite, and nitrate but a good kit that also includes pH/alkalinity would probably be a better deal)
-Thermometer (a cheapo glass one is fine)
-Gravel vacuum (unless you have one already?), doesn't need to be fancy or electronic
-Substrate (avoid small gravel, goldfish are notorious for accidentally...well, sometimes purposely, ingesting substrate)
-Light (this is really just for you to be able to see the fish and also to grow plants...a couple of fluorescent tubes would be fine)
-Rocks/other decorations (make sure any caves don't have small holes that a fish could get stuck in, goldfish like to do that, they're very curious)
-Air stone and air pump (whatever model you want, this is just going to provide some additional aeration to keep the water oxygenated and moving...I like the ones that have a battery back-up in case of a power outage)
-Dechlorinator (you probably already have something like this but I'm adding it just in case! Prime is my favorite but other brands like AquaSafe work just as well)
-Tetra SafeStart (this isn't necessary but I think it'll make things MUCH easier for you if you can kick-start the cycle and move your fish asap)
P.S. Please don't feel like you're a "terrible fish mommy"! The fact that you care enough to change things and save your fish proves otherwise!