High-End Custom PC Build for Web Designer

For photoshop or illustrator, gpu does very little for the most part. There are a few tasks which can make use of gpgpu acceleration but even a mid range card like an rx 480 or gtx 1060 would be more than enough. Ram is important as is the drive setup. Make sure to have several drives, one for the os/apps, perhaps a smaller ssd for a scratch disk, another to hold the saved/working files. 16-32gb of ram would be ideal.

While a 4k monitor may be nice for overall screen clarity keep in mind that web design is often somewhat generic. Ie the graphics used won't be displayed on 4k monitors for the most part, they should look good on a variety of resolutions, 720p, 1080p etc. Basically while using a 4k monitor is nice for the designer, you're not going to be making 4k hi res graphics.

Many graphics will actually be quite plain, small, low file size. A wall makes for a nice amount of room to paint a picture but when designing postage stamps it won't matter as much. Even on a 1080p monitor If I'm running out of room in photoshop to design a web graphic it's going to be humongous and ridiculous. Graphics themselves in web design have largely been moved away from for years, no need to draw a huge sidebar. Not when you can create a small 4x4 px colored 'dot' and tile it and even then much of that sort of thing went by the wayside years ago with css. The reason I even mention this is because all this monster processing horsepower you're planning on would make more difference in print design. When applying effects to or rotating, working with large images like 4-5,000x 4-5,000 pixels. Not with 400x150 px web graphics. If your web graphics for a web site are 4,000x5,000 px you're way off base.

Also a mobile version wouldn't be a bad idea depending on the function of the site. In other words the final output aren't going to be massive mona lisa wallpapers and what looks good in a larger pixel count with smoother lines on a higher res 4k display may look like garbage on a 720p display. Bandwidth plays a role, simpler graphics use less bandwidth. Better for the webhost, better for the end user.

Most tasks in photoshop are not only cpu based, they're mostly single threaded. No need for an 8 or 10 core 'beast'. Something like an i7 6700k should work well. Strong single core performance. Much like a ferrari vs a hummer, in a street race on pavement, the ferrari. Offroad, the hummer. The right tool for the right job, simply throwing buckets of money at it won't make it necessarily run better.

https://www.pugetsystems.com/labs/articles/Adobe-Photoshop-CC-Multi-Core-Performance-625/