Is the HP Pro Mini 400 G9 good for everyday office work?

Yes. The i7-12700T and 16GB of RAM handle email, browsers, spreadsheets and video calls smoothly. It is not built for gaming or heavy GPU work.

Can I upgrade the memory and storage?

HP's Pro Mini line typically allows memory and SSD upgrades, but the exact slots and limits depend on the configuration. Check the service manual for your specific model before buying parts.

How does it compare with a full-size desktop?

It uses a 35W T-series processor, so it runs cooler and quieter but slower under sustained load than a full-power desktop chip. It also has fewer expansion slots.

Does it come with a monitor, keyboard or mouse?

No. This is a desktop PC only, so you need to supply your own monitor, keyboard and mouse.

Is 256GB of storage enough?

It is fine for basic office software and documents, but it fills quickly with large files, games or many applications. Plan on adding storage or using external drives.