Hi @GlenNinja, mate you have done well to have an MPX as your first blaster, believe me you just avoided a lot of heart ache from purchasing some of the other duds out there.
The MPX uses the Gen9 t-piece so any outer barrel which fits this t-piece will fit the metal receiver. These barrels only need the slot in the top and bottom. Also you have to think about what silencer you want on the pointy end of the outer barrel. Just grub screwed onto the outer 19mm or reverse threaded like the standard MPX. You also have to take into consideration if say you want a 10 inch hand guard, how much if any outer barrel you want protruding from the hand guard.
I personally don’t like any barrel showing, just silencer going straight into the hand guard.
Here is a mock I just did with my MPX for you with a spare 10 inch hand guard.
Here is a pic of the standard barrel in your MPX, its a great barrel showing it has many slots in the end but you only need a barrel with the bottom and top slot to take the gen9 t-piece.
If you get a hand guard just make sure it has a barrel nut like this. Look up gen9 compatible ones they all fit.
Here is a link to a suitable outer barrel.