Just curious. Which mods that you're giving advice on have you done to your Mustang? Photos please. We like to see different configurations.I was in a similar position not long ago and ended up selecting the 2018 Ford Mustang Ecoboost. It's much better balanced than the GT and lighter weight. Particularly if you're coming from a german/mid-engined car, it will be more familiar. The only downside is obviously less power (which you mentioned you don't mind).
Yes you'll want to change rims, and they're called wheels. You'll need some upgraded tires as well.
You could go auto or manual, if you go auto you may need additional transmission cooling. If you go manual, I'd personally go for one that comes with downshift rev-matching, unless your heel-toe (not heal/toe, by the way) technique is perfect.
No, none of the seats that come any of the packages are worth it because they aren't SFI/FIA certified and they won't support a 6-point harness. You can't run "4 or 5 racing belts", almost any track or racing series will not allow a 4-point belt. I don't know what you mean about "putting a bar" in it, don't install a harness bar. You will want at least a rear 4-point cage, or a full cage if you are doing any wheel to wheel racing.