Yeah, you certainly want to do the RAM yourself. You can get 32gb of Corsair RAM for less than half the price Apple is charging in the Mini. If you get at least the $1100 model, you're getting 256GB of SSD anyway, which is probably ample, any working files are better used via an external storage solution, the Thunderbolt 3 makes that more than adequate, so the standard SSD should be fine.
A Razer Core X is around $300, you can slap in even a $300 GPU and it's going to massively outperform the top MacBook Pro for considerably less.
Software has to specifically allow for eGPUs, but support is seemingly good and only getting better. Should meet your needs well. If you favour the i7 over the i5 though, I really don't know. We should know the thermal performance next week, but I'd imagine it's robust. So the CPU is kind of a coin flip at this point, depends on your budget after the other stuff. But you should be able to get the standard $1100 package, 32GB of RAM, the Core X, and an RX 580 for around two grand if you shop around.
