@essiw Same as you, I'm in the market for an eGPU solution. Been researching this for a few days now, mainly on epu.io
You have two options: buy an "all-in-one" solution such as the Blackmagicdesign eGPU (expensive), or "build" one yourself: buy an enclosure & stick a graphics card in. Cheaper, and easy to do.
If you don't need a high-performance graphics card, but a more 'standard' card such as an AMD 580, there's no need for a Razer Core X enclosure. The Sonnet Breakaway 350 (for example) is cheaper, and they're easy to find used on eBay.
For the actual graphics card (as already mentioned by others), you'll have to stick with AMD. Apple doesn't officially support NVIDIA (though this may change again at some point, who knows...).
An AMD 580 is probably a cheap but decent all-round solution. On Jan 21st, AMD will start selling their latest AMD 5600, for about $280.
Take a look at the forums on epgu.io though... there are some things to consider when working with eGPU on a Mac. The latest thing in terms of "weird behaviour" is this, for example
Hi, I am hoping someone can help me. I have the latest Mac mini (2018) hooked up with an external GPU (Vega 64) running a thunderbolt 3 con...
egpu.io
Not a big problem, but something to be aware of.