Really depends what you have now and what you are looking to achieve. The “core 3” i call it, are mobo, ram, and cpu that needs to be generationally compatible. These tend to but not always be purchased at the same time.
If you are keeping your C drive, then no, you dont have to reinstall everythimg. Windows licensing is weird now. I think its supposed to be transferrable if you disable the previous computer since it is account bound. Someone else will need you comment on that. That is a ‘legit’ license issue. Most other software wont care if your cpu or mobo changes.
If windows is installed on a harddrive and not an ssd, then i would strongly reccomend you do a fresh install on a new ssd. You can plug in your old drive after install to transfer files. Then wipe it and use as storage.
As long as you pick a compatible mobo. Theres not really any difference between them for most people. Mostly just IO differences.
Also remember that cables… are just cables. They will take whatever connector you put on thebend of it. So you can use a different one 12v so you dont accidentaly plug things in wrong.
But if you need to pull dc out of POE, you can get thigs that do just that. Search for POE extract or splitter. Many will let you pick wjat voltage to get out of it.
Imo youre better off running cat6 or 6a everywhere (better yet conduit where reasonable) and not running a 2 conductor pair along with your cat cable. Just terminate your cat cable as usb if you dont want to use an extractor.