I went to the Apple site and configured this:
3.2GHz 6‑core 8th‑generation Intel Core i7 (Turbo Boost up to 4.6GHz)
16GB 2666MHz DDR4
Intel UHD Graphics 630
512GB SSD storage
Gigabit Ethernet (10/100/1000BASE-T Gigabit Ethernet using RJ-45 connector)
Find a couple nice 27" HD monitors and keyboard/mouse/touchpad and you'd have a nice setup for about $2k.
If possible I'd get 32GB RAM to help store more GIS app working information in memory, and for longevity. If budget is a factor then as Lee_Bo mentions above, 16GB is probably OK. Since apparently the new Mac mini are RAM up-gradable, you could add more RAM later if you start to notice Memory pressure and swapping.
You've probably already confirmed this, but if not, suggest checking with the GIS app developer to ensure it is supported for MacOS, along with the other app requirements.