I'm not sure what the deal with the MgSO4 (magnesium sulfate) is. That's generally included in GH Booster, and will help increase water hardness. It can also be bought as epsom salt, by the way. You're missing the "P", which is what you noted - the KH2PO4 (monopotassium phosphate). If anything, you might be able to do without the K2SO4 (potassium sulfate), because potassium is included in the KH2PO4 ("P") and KNO3 (potassium nitrate, "N"). GLA has a package that includes the basic NPK+Trace, or you can buy separately. If you want a basic package, just make sure it includes:
When you buy it, be aware that basic EI dosing calls for using pretty much equal amounts of the last 3 on the list, but 4 times as much KNO3 as each of the others. Keep that in mind when deciding what ratios to buy them in. You will likely tweak the quantities over time, but that's the basic starting point.
Side note: Sometimes GH Booster is substituted for K2SO4. GH Booster has some K2SO4 in it along with other water hardeners (calcium, magnesium, manganese).