Is the HP Z2 Mini G1i good for CAD and 3D work?

Yes for mainstream CAD, BIM and 2D/3D design. The RTX A1000 is certified for professional applications, but it is an entry-level card, so very heavy rendering or large assemblies will want a bigger workstation GPU.

Can I upgrade the RAM and storage?

HP workstations typically allow memory and SSD upgrades, but the Mini's compact chassis limits you to SO-DIMM memory and a single M.2 slot. Check the exact configuration before buying if you plan to expand later.

How does it compare with a regular mini PC?

It is larger and more expensive than consumer mini PCs, but it offers workstation-grade components, ISV certification, vPro manageability and a discrete Nvidia GPU that most mini PCs lack.

Does it come with a monitor and keyboard?

No. It is sold as the desktop unit only, so you need to supply your own monitor, keyboard and mouse.

Is it quiet enough for an office?

The small chassis uses a blower-style cooler that stays fairly quiet at idle and moderate loads, but the fan becomes audible during sustained CPU and GPU work.