What are the minimum FPGA feature requirements for decoding and encoding 4k60 HDMI signal
I think you are going about this all wrong. As it is obvious that you are just now entering this area of engineering the first step should be to do work to build a working model or prototype so that you can learn about the tools, algorithms, intellectual property (IP) development and open source IP. If you choose to go the FPGA route you should select a development platform that does not fetter your ability to achieve the end result.

Once you become well versed in the technology and have built a working model then you can begin to think about fitting that to the  most cost effective solution. The algorithms that will need to be developed should be portable if you develop them in a hardware description language (HDL).

Make no mistake, the project you are talking about will not be an "easy task done over some weekend". This is going to be a huge opportunity to learn about technology and how to tune that knowledge into a working application.