See above.  You should carefully size up the internals --  the cables for external GPUs are generally hefty and it's more likely than not you'll need to leave the back off the system. Also realize that a wifi card is wired for a device that needs perhaps 200 megabits a second.  The slot may offer 300 Mbps -- but a PCIe x16 GPU card needs up to 16 gigabits/sec -- so the card will be severely throttled to just a couple of percent of what it needs by the adapter. Practical eGPUs need Thunderbolt (or some other proprietary high bandwidth interface such as the old alienware graphics amplifier) to work anywhere near the full potential of the GPU card -- your system has neither prerequisite.      
I heard you can somehow get a gpu conneted by getting a gpu to hdmi to m.2 but would it work on a dell xps 18
Would an external gpu work through mini m.2