Is the ThinkCentre M800 good for gaming?

No. It uses integrated Intel HD Graphics, which handles video and office work but not modern games. A dedicated graphics card is not part of this configuration.

Can I upgrade the RAM and storage?

Yes. The small-form-factor case has room to add memory and swap the 1TB hard drive for an SSD, which is the single biggest speed improvement you can make.

Does it come with a monitor, keyboard or mouse?

The listing describes the desktop itself. Treat the monitor, keyboard and mouse as separate purchases unless a bundle is stated where you buy.

What ports does it have?

It offers USB 3.0 ports plus HDMI and VGA outputs, so you can connect a monitor and common peripherals without adapters.

Is it fast enough for office work?

Yes for documents, email, browsing and video calls. The Core i5 and 8GB RAM are fine for that, though the hard drive makes startup slower than an SSD-based PC.

How does it compare with a mini PC?

It is larger but easier to service and upgrade, and it includes TPM security. Mini PCs are smaller and often faster with SSDs, but offer fewer upgrade options.