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First off, I have been on this board for over 2 years and still do not know how to do searches that produce results. Often it is better to let Google search on the web to find the results on the board.
As far as I know, all of the stock versions since v4 have included CAT capabilities. I do not have version 6 so I cannot give you specifics for that version. I do use the CAT capability for my v4 and v5 rigs. You can use Yeasu FT-817 or FT-857. Both have worked for me with digital mode (WSJT-X) software. It works with CAT PTT, 38400 baud, 8 bits, and defaults for others.
You can get the source code for the stock firmware (latest) from Ashhar Farhan's GitHub account. He is the author and primary designer of the ubitx. Here is the link:
As far as upgrades, there are too many to enumerate them all. If you have already ordered/received a ubitx, then the first step is to get it working. After that, it will depend on what you want to do with the rig. There are mods that fix the shortcomings for CW, others that are focused on the digital modes, and still others that open up enhanced software capabilities that would be up to you to program and use. All are valide depending on what you want to do. There is another thread where I listed what I would do if I bought a new v6:
One place to start for possible upgrades might be ubitx.net. That site seems to have the most information on the past mods as well as possible changes. Another is HamsKey.co, the blog of the KD8CEC author.
The ubitx, like this board, has huge possibilities. It does take some research and forethought to make it what you want it to be. For the price, there is nothing else that I know of for multi-band SSB kits. There are some that have been promised, however, they are still not available. No date has been announced yet.
Above are my thoughts based on my experiences. Yours may and can be different.
Enjoy the hobby!