CPU is going to be soldered on this one, so get the one you want as there’s no way for you as an end user to change your mind after the DOA warranty economically. The 4XXXU Haswell laptop chips are BGA only - that has been the case for many years with low power SKUs, even before Intel split them by adding the U.
Max out the RAM and put an SSD in. As far as installing the parts, refer to this guide. For RAM, you need DDR3L - the Haswell CPUs require DDR3L and cap at 16GB (i3/i5) or 32GB (i7). That said, the laptop is probably only validated for 16GB, but it’s new enough (PCH based) that there’s no BIOS imposed limit so there’s a good chance 32GB will run stable. You’re still gambling there to a point, but unless there’s a design flaw with your specific board you’ll be in good shape.
Better to upgrade HD to SSD.