There are two ways you can go about this, and it all comes down to preference. When you start the game, by default, you cannot equip six different gadgets. The two options you have to increase how many gadgets you carry is by either going down the Technical Aptitude and getting each version of the Gadgeteer, or by wearing heavier armor. Most armor will give you an extra gadget slot, you'll have to buy a tactical vest if you want to get the 6 different gadget spots you need. If you do not invest at all into the Technical Aptitude skill tree, you will need to buy the Tactical Utility Armor for $50,000.
Once that's done, just buy any gadget you can. Price doesn't matter as long as each gadget is different, so buy whatever you want. Offensive gadgets don't even need to hit their targets, so long as they are thrown. Also, Sound Generators can be used an infinite amount of times, as long as you have one.
Make sure that you do not try to do this on any of the missions listed in One With the Shadows if you plan on getting that achievement, as you will alert nearby enemies.