Is the Custom Mantis Desktop worth it over a brand-name prebuilt?

It depends on the parts you configure. A brand-name prebuilt gives you a fixed, widely reviewed spec; the Mantis gives you a chosen parts list assembled and tested. Compare the exact components against a similarly equipped prebuilt before deciding.

Can I upgrade it later?

It is a desktop tower, so standard internal components such as graphics cards, memory and storage can generally be swapped. Check the specific motherboard, power supply and case in your configuration, since those limit what fits.

What comes in the box?

You get the assembled desktop and its power cable. Monitor, keyboard and mouse are not included unless you add them to the configuration.

Does it come with Windows installed?

Confirm the operating system in your configuration before ordering. Custom builds may ship with or without Windows depending on what you select.

Is it good for 1440p or 4K gaming?

That depends on the graphics card you choose. A mid-range card suits 1440p, while 4K needs a high-end GPU. Match the card to your monitor before configuring.