Desktop HD is the way to go!
I have seen several people here on DDJT that suffers from problems with external HDD
that goes away when putting the soundlib. in the internal HDD.
If you are to play outside your bedroom you will have one device less to bring
and less cables….
IMO no more than max 60-64Gb will be needed for music if you clean out everything that you will not ever play and
clean out old tunes when bringing in new ones on a regular basis.
But you will need an external HDD to back up your laptop on!!!!
That is a very important thing, always do backups!