Don't travel on an uncomfortable bike - no one enjoys it!
That is especially important with a pillion!
Roger2002's ideal of a test ride is good - do it! Even if it is only an overnight - travel Saturday out, overnight somewhere, return Sunday. That will give an impression (pun intended) as to comfort being acceptable.
You may find the seat is not acceptable - possibly a replacement seat from a wrecker might be a way to go, may look not so good but comfort is what you are after. Once you have changed it test it with another test ride as above. You may find that just half a day into the Saturday start that things are not acceptable, don't persist return and change it. For the pillion you may find a wide seat is best - look for them. The key for the pillion is them being happy, don't go too far into unpleasant experiences otherwise that pillion may never go again
As for packing - put out what you think you'll need - take half of it. If you really need something you don't have - buy it.