Wheeee! A 75 gallon tank? Thats HUGE! You could prob fit 4 goldfish in there comfortably
I agree, definitely need a filter. I use the ones that come with a bio-wheel (Penguin, Emperor, all made by Marineland). I like the bio-wheel as it gives the nitrifying bacteria an extra place to live, which means you can gravel vac the bottom of the tank to your heart's content and not mess up the bio-filter and the cycle (and you will need to with goldfish...messy buggers). If you are unfamiliar with the nitrogen cycle, have a gander at this: http://faq.thekrib.com/begin-cycling.html
. There is other awesome info on that site; click on "main map" at the bottom to see.
I strongly suggest you not use an underground filter. They clog easily and again, goldfish are messy. A hang on back one should be fine, and since it moves the surface, you won't need an airstone and pump (those bubbles don't put air in the water; they move the surface so oxygen can be exchanged up there). This way you kill 2 birds with one stone and save a couple bucks.
Live plants are not necessary; in fact goldfish are notorious for ripping them up LOL
I'd go with plastic ones. Easier to take care of, and they don't die.
looks like you need a HOB
(hang on the back filter), some plastic plants, maybe some gravel and you WILL need a gravel vac. Not expensive, and you'll need to get up all the poo from the bottom. Oh. and food (which I'm guessing you have already). The only major expense I see is the filter which can run you from about $50- $100 depending on type and where you get it.
I also suggest educating both boys on the basics of fish care; 8 year olds suck up info like no one I know, handicapped or not (I work with disabled kiddies). They can certainly understand the basics of the cycle and you get to use words like pee and poo
Be warned. Fish keeping is highly addictive. *grin*