The ex-Alibaba KU3P boards are available for under $200, although you do need a separate JTAG cable with flying leads to load designs on them. I recommend starting with that. I'm building the next-generation version of Corundum on those boards, at least initially. I will certainly be adding support for Alveos and such in the near future. As far as adding functionality, it depends on what you want to do, but in general you should expect to have to do a fair amount of tinkering with the design.
This is correct, and the result of those streams has been released as corundum-proto here: https://github.com/fpganinja/taxi/tree/master/src/cndm_proto . Note that this simplified design is intended for educational purposes only, the "production" variants will be much more capable (corundum-micro, corundum-lite, and corundum-ng).
I'm using some very cheap (~$200) ex-Alibaba FPGA boards for the initial development of the next-generation version. They have 8 lanes of PCIe gen 3 and two SFP28 capable of operation at 25 Gbps. Honestly that's pretty hard to beat. Support for other boards will be added in the near future. The new library will also include a new IP stack called Zircon, which supports UDP, among other things.
The GPU uses https://github.com/alexforencich/verilog-pcie + the Xilinx PCIe hard IP core. When using the device-independent DMA engine, that library supports both Xilinx and Intel FPGAs.