IOS – What will registering an apple device a developer device do to it

That totally depends on the apps you want to do.

Do they need GPS localization? A microphone? Front or back cameras? Hi-res camera?

Besides, you can't test an iPad app on the iPhone, so you'll need an iPad no matter what.

Edit: According to the comment, an iPod Touch would be enough for you.

Each year when you renew your program membership you can remove devices added in previous years. So if you had 100 devices allocated, you could delete 5 and add 5 new devices.

As soon as you add a new device, you can no longer remove devices for the remainder of that year.

So what they are telling you is each year, after renewal, delete any devices you need to remove first. Once you add your first new device, you are only permitted to add devices until your next renewal up to a maximum of 100 devices.