Hi Guillaume,
to most people the Sire U5 will be good as it is without any work by a professional.
If you are a nitpicker like me you might want to upgrade the tuners and pickups at some point, but that's a very personal thing and mostly a matter of taste (I even mod >2000Euro basses that already have quality hardware with other quality hardware, just because I prefer a certain brand of tuners and/or pickups.. I just like to work on instruments in general, so a lot of the mods I do are not so much out of neccesity, but rather to give my personal flair to the instrument...).
The gigbag I use for my U5 and other short scales is this one (a perfect fit!):
https://www.thomann.de/be/thomann_gigbag_deluxe_egitarre.htm
As to the Labella strings I prefer to use the La Bella 760FL-S Deep Talkin Bass shortscale flatwound set on my short scale basses as they have just the right amount of tension to compensate for the sloppiness that often comes with the shorter scale, yet they play effortlesly and have a great sound to them. Again, this is very subjective, but if you like playing on flatwound strings you can't go wrong with Labella i.m.o.
https://www.thomann.de/be/la_bella_760fl_s_deep_talkin_bass.htm
Hope you enjoy your new bass!
Kind regards,
Don