I'm a non-pro who enjoys working with After Effects. What are your recommendations for a desktop PC?

If you're in the UK, I used these guys after a reccomendation:

https://www.novatech.co.uk/workstation/media-and-entertainment/video-editing-and-post-production/

Their workstations are not exactly cheap, but they worked with me to get a PC in my budget.

Entirely depends on what you do and what you can afford. Motion graphics have different priorities than VFX work and there's all flavors inbetween like not even caring about actual rendering performance when merely using AE as a crutch to create web content exported with Lottie/ Bodymovin. You really have to be more specific for reasonably exact advice. That said, these days there's more or less only three types of computers: low range portables, mid-range semi-stationary notebooks and mini desktop computers and anything above that almost automaticalyl counts as a high-end workstation desktop or gaming computer. of course the devil's in the details, but you can be pretty sure that even a 1500 USD computer these days has enough juice to handle AE, give or take the bugs and issues that may need to be considered and that may limit hardware selection.

Mylenium

This really depends on your budget. Also, are you going to build it or do you want a pre-built? I assume you are looking for a pre-built, so I'd recommend this one:

https://www.hp.com/us-en/shop/pdp/hp-pavilion-gaming-desktop-tg01-2285t-bundle-pc

For a pre-built, here's a quick PCPartPicker:

https://pcpartpicker.com/list/xrkH4s (Not the best, I just put this together in like 5 minutes. You should get an understanding of what you might want though)