Is the Precision 3591 good for CAD and 3D work?

Yes. It pairs an Intel CPU with NVIDIA discrete graphics, which is what CAD and 3D apps need. Check which NVIDIA GPU the listing ships with, since configurations vary.

Can I upgrade the RAM and storage?

Dell supports up to 64 GB of RAM, so you can add memory beyond the installed 16 GB. The 512 GB SSD can also be swapped for a larger drive.

How does it compare with the Precision 3581?

Both are 15.6-inch Precision laptops with NVIDIA graphics. The 3591 is the newer generation, so expect updated Intel CPUs; confirm the exact chip and GPU in the listing before buying.

Is the screen good enough for photo and video editing?

The 1080p matte panel is fine for everyday editing and office work, but at 60Hz and standard resolution it is not a color-critical or high-refresh display. Serious color work would want a higher-end panel.

What ports does it have?

It has USB-A, USB-C, HDMI, Ethernet and a 3.5 mm headphone jack, so you can connect monitors and wired networks without a dock.

When is the best time to buy it?

Workstation prices move with Dell promotions and back-to-school and holiday sales. Compare the three new listings and check the price history before buying.