How to stream video (and process) into FPGA

GigE Vision isn't that bad (I've used it, in Cyclone's). As Tricky mentioned, it's UDP based. Which means all you need is a board with the GigE brought to the FPGA fabric (not the ARM HPS side, if there is one), and then some elbow grease using the UDP Offload Example as a starting point.

http://www.alterawiki.com/wiki/nios_ii_udp_offload_example

You should be able to use PC software to e.g. configure the camera and command it to start transmitting to a destination IP address that the FPGA is listening on.

Like I said, >900Mb/s is fine given no other network activity. Point-to-point between the camera and the FPGA, your 800Mb/s is very do-able. As Tricky mentioned, if you are buffering into DRAM then memory interface design may be a concern. "frame subtraction" in real-time requires 2x the pixel rate (storing frame (N), recovering frame (N-1)).

The DECA board can be used to learn about video processing in FPGA, which you have some learning to do independently of USB3 vs. GigE Vision vs. CameraLink vs. MIPI; the kit looks "nice" because they give you MIPI camera in -> HDMI display out lab for you to work through.

Unfortunately, the board doesn't have GigE so you can't also use the board for any GigE Vision work. The Enclustra Mercury SA1 might be a good fit, with their PCIe card carrier for your one-off design.

https://www.enclustra.com/en/products/system-on-chip-modules/mercury-sa1/

Your project final board might reduce down to a custom carrier for the Mercury module, which just has one or two GigE PHY, power, etc.

Your thesis algorithm can be roughed out entirely on the DECA board with whatever frame rate limitation it has, and from there you should be able to understand the FPGA requirements. After you have structured your IP block like (Avalon-ST VIP protocol input -> algorithm -> Avalon-ST VIP output) [or AXI-S] then moving your work to another board with another camera interface isn't a big deal.

FPGA component cost-wise, in your shoes I would probably target Cyclone 10 LP in -040 or -055 size grades which Digikey has for $40 to $70 qty 1. This is all assuming your algorithm isn't a can of worms and it's really as simple as "frame subtraction" as you mentioned in your opening post, and you might want to double check your estimated resources before buying anything.