Question about Pro-Ject Phono Box

No, it's included but there is no off switch to the item; that said it's a big jump in sound quality and worth the investment.

You do not need to buy a power adapter separately, as it is included in the package.

No. It's all in the box. In retrospect I would have bought the next model with adjustable in/out impedance. Combining stuff might get tricky.