I will not be able to answer your questions but vBulletin 5 only has 53 released add-ons, compared to almost 2000 of vBulletin 4. It's more work to customize vBulletin 5 and has its limitations when it comes to that.
The code itself may be up to the current standards but for you, as a customer, it doesn't matter. It needs to work well for you. You will definitely not be able to use your current vBulletin 4 skin for vBulletin 5, someone will have to remake it from scratch.
If you want my personal opinion: don't do it. It will be too much work and a waste of time.
Someone else made a thread about this recently with bad experience:
https://vborg.vbsupport.ru/showthread.php?t=315689