If you are worried about the backup, using an external drive makes a lot of sense. Not to be overcautios but you could install the new version to a client, then restore to it to see how it works. Finally, you could get AC support to do it for you. It isn't hard. I don't want to scare you. I can't believe it takes up 100GBs.
You could go to Add/Remove programs and delete programs you don't need. Delete your temp files. I am sure there are many files and programs you could delete.
