From the ingredients listed, I see nothing that I would be concerned about. Since it is more of a fertilizer, I wouldn't use it as a main substrate. There is no reason to toss everything except the Miracle Gro. A simple thorough rinse should be sufficient.
Don't take someones word for contents of a product you know nothing about. Look it up and failing that, go back to the store and read from a bag on the shelf. Salt is an over-abused commodity in the aquarium world and except for treatment, is unneeded.
Take your time to reasearch and learn BEFORE setting up a tank. List your ideas on a paper and then find out if it works before committing any money on it. 90% of aqaurium plants are nothing more than bog (swamp) plants that grow well in aquariums, with many being salt-sensitive. Bleaching plants is also unneeded unless removing chance parasites, free-loaders or bacteria. It needs to be done with care as bleach will kill most all of them. Have a reason to bleach other than someone claiming it removes ferts and copper (simple rinsing can do that). I bleach all incoming plants before putting them in my tanks. I use 19 parts water and 1 part bleach. I soak the plants in it for 3 minutes and then rinse in cold clean water. I make sure I get the bleach off every leaf and roots. A little bleach goes a very long way.
Never accept someone's advice if they can't tell you how or why to do it. I am NOT pointing a finger at anyone, but even here you can get bogus information.