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What he said. An inch of so of free axle is ok. But don't have your drive wheel on the end of a long free axle. It will vibrate badly, and make it harder to e'g work with a tool rest.

Keep in mind that the tension in a spring is a linear function of its compression, so you'll have to some fine tuning to make sure that when you are using it, the compression is such that that is the...

Nothing too flimsy I'd say. Take a look at the KMG ones, and then pick something close.

The drive wheels for my grinders are mounted directly on the motor.
The motors are powered by a VFD so pulleys are not necessary.

There are many different sizes. If you're going to make knives, I suggest you buy a pair of calipers.
they're cheap, and you will be able to tell exactly how thick or wide a piece of metal is....

1.5 HP is good enough. I have one, and it has never bogged down, even at low speed.

Unless you can shield it, I advise against it. Grinding throws around a ton of metal dust. An open motor with high currents is a gigantic short circuit waiting to happen. Motors like that are meant...