Extra extractor, a set of rollers, a couple of roller retainer plates and three roller retainer pins are always a good idea. To which you would need the punch and a small hammer. At this point could consider a firing pin and firing pin spring, plus possible locking piece, recoil spring assembley, ejector, ejector spring and axial pin. Next step is a bolt group ... sky's the limit ;-).
What size Pelican cases depends in part on how much ammo you are including with the soft case and MP5, less it easier to carry but one hates to be at zero ;-). If it was me I would pack the ammo on stripper clips and include a couple loaders for the 36 round stripper clips.
The HK cleaning kit (looks like the M16 kit) is a good start, for supplies add some type of CLP (Break Free use to be specified as an alternate in the manual), along with a supply of patches. Could include an HK sight tool, however, in theory you already have the MP5 sighted in (I like the 5" hemostats as they are flat, fit in the tool kit and also work to pick/hold thinks like the roll pin for the roller retainer plate).