If you're looking to add an expansion drive, I'd strongly recommend getting an m.2 instead of a 2.5" SSD. When removing the bottom cover, you'll see the pre-installed m.2 that MSI includes, and then an empty expansion slot for another m.2. The rest of the internals are covered with that "heat shield" plastic barrier, which you'd need to remove to access anything else, so I'm not even sure if there IS a space for a 2.5" drive under that. There are videos on YT on how to remove the barrier, but use caution.
The model I purchased came with a 1TB m.2, and then there was an empty m.2 expansion slot available, so I ended up installing a Sabrent 2TB NVME m.2 that I got on sale. If you just want a data drive, just about any NVME m.2 is fine. If you need high performance for large file transfers (like with photo or video editing) then you may want to consider going with a Samsung EVO or PRO m.2, as they seem to be the premium model of choice for most professionals. But again, if you're just needing the expansion drive for casual use, or gaming, any NVME m.2 within your budget is fine.
Something to note is that MSI partitions the included 1TB m.2 at the factory into a boot drive (about 590gb), a recovery partition, and then a 350gb data drive. I didn't really care for this, because the recovery partition physically exists between the boot partition and the data partition. This means I could not recombine the boot and data partitions back into 1 logical without some partition trickery and using 3rd party software. In the end, I didn't bother -- I just left it as is, and I used the Sabrent 2TB as my data drive.