Wellllllll depends on use case.
ATM Windows updates have been playing silly buggers causing a shit load of problems with AMD cards. If you stick with Nvidia you'll be fine.
I'm kinda eyeing up a Mac mini and a Nvidia eGPU in the future as I don't need GPU power in macOS. I could waffle for ages about this, so let me just try and fire out some bullet points.
CPU choice: Stick with i5 as thermals on the mini cause throttling on the i7 and gaming performance (if you care) will see minimal gains.
AMD cards are harder to set up and Navi drivers are a bit shit still, as well as there being issues with hardware failures.
The latest builds of Windows cause issues, stick with pre 1903 if you want an easy life, some people stress updates due to security patches but if you are only using W10 for games who cares? My Macbook is running 1809 and its solid.
I've just built a SFFPC because I dont fancy buying any of the current Mac models (Mac mini may be updated in March) and I kinda like it, for now, I'm starting to try and hack it but its AMD so a whole new world, my last Hackintosh was an intel machine (8th gen maybe?) and it was super quick and easy to set up and rock solid. I had to go AMD this time as it was the only way to get a DP in to TB3 out mobo so I could power my first gen 5K ultra fine, which is does grandly (Oh hello PUBG in 4K).
If you are The London based and not a dickhead slide into my DM's as my eGPU is going spare for a few months and I'd be happy to set it up for you to check you can get things working as you hope. Also egpu.io
Of course, you could always just buy an iPad.