Is the RTX 5070 enough for 4K gaming?

It is a 1440p-first card. At 4K you will need DLSS upscaling and lowered settings in demanding titles. For high-refresh 1440p it is a good fit.

Can I upgrade the RAM and storage?

Yes. It uses standard DDR5 memory and M.2 or SATA storage, so adding RAM or a second drive is straightforward.

Does the Ryzen 9 9900X help with streaming?

Yes. Its 12 cores let you game and encode a stream on the same machine without a big frame-rate hit.

Is Wi-Fi 6E worth using here?

If your router supports 6 GHz, Wi-Fi 6E gives lower latency and less congestion. Otherwise wired Ethernet is still the better choice for competitive play.

What comes in the box?

The tower, a power cable and documentation. A monitor, keyboard and mouse are not included.

How does it compare with building your own?

You pay a premium for assembly and warranty, but you avoid part-hunting and compatibility risk. Builders can usually get better cooling for the same parts.