Ditch the dock and go this route instead. The expresscard to PCIE adapter gives you PCIe x2 speeds instead of just x1, and unlike the dock solution it sends the external GPU's I/O back to the motherboard so you can use the laptop screen instead of being forced to connect to an external monitor. If you decide on an nvidia gpu, make sure to use this script so the card actually works.

You can use any GPU you want, right up to the 4080. But you have to decide where to draw the line between price and performance just like if you were building a desktop computer.

• GTX 1060 Refurb $140

• GTX 1070ti Refurb $157

• GTX 1080 Refurb $173

• GTX 1080ti Refurb $243

Pick one. If it were me, I'd probably get the 1080ti just for shits and gigs. Be sure to get a PSU with enough wattage to power the GPU under full load. 500w is probably good.