The Pre-Amplifier

If your audio interface has no pre-amplifier or just one of poor quality, it might be a good idea to buy an external one.

As for all other audio tools: the jungle of available gear is infinite. The higher the price, the better the quality, but keep in mind that you're building a home studio, so it would be enough to buy a pre-amplifier that can adjust the gain and output level.

I am using a tube pre-amp which gives the sound some warm touch, but this is not necessary at all. It's a cool feature if your pre-amplifier has the ability to apply effects, but all effects can also be done after recording with plugins in your DAW. The latter one is often the more inexpensive way.