What Epsom salt and what quantity to buy? Depends on what do you plan to use it for..
When buying Epsom salt for bath use, bear in mind that the bath should be ideally repeated 3 times per week in order to enjoy the full benefits of Epsom salt soak. To a bath tube of regular size, 2 cups of salt (400 to 600 g) should be added. So if you intend to soak in Epsom salt regularly, you will need at least 5-7 kg of Epsom salt per month. You can choose a pure Epsom salt – this is the most economic option, or you can choose one of the many scented salts that are in general more expensive.
If you intend to use Epsom salt as laxative, you’d better search for a product that is specifically for internal use and give you clear dosage instruction so that you do not overdose.