The Artix-7 is an excellent series of devices for learning how to work with PCIe. You're limited to Gen2 speeds, but that's not an issue for developing most applications. One thing to keep in mind is that "Artix 7" is an entire product line of devices, not a single chip -- there's a spread of "amount of logic resources" with prices to match.

Also, the whole Artix 7 line is supported under the free licensing, so you can actually get started and learn a lot even before you have a development board. You should do this! You'll find that you can develop your design and test it in simulation before you ever touch hardware, and this also helps you plan for exactly what hardware you'll need...