This is a complicated question with many parts. I doubt I can cover all of the issues comprehensively here.
A few things I recommend:
1. Avoid using referenced files with Photos, unless you are absolutely certain that you will NEVER move the files again. Photos has pathetic referenced file support – moving files is a one-at-a-time activity. If you have an Aperture library, consolidate the images before moving to Photos, or moving them all to a giant drive.
2. Make sure all of your images are online and have full-size previews made for them. That’s important because Photos will not copy your adjustment data over – just the preview. It will try to make a preview for you, but there are bugs in the implementation.
3. Keywords are copied over. Ratings are turned into keywords.
4. Photos doesn’t have projects, so you need to make albums. It also doesn’t allow a lot of the complex nesting that Aperture did.
5. Photos is superior to the Finder if you want to have albums, keywords, smart albums etc. If you have a strict organizational system and can stick with it, then the Finder is good for that.