My first keezer was a 5 cu ft. With a 10" collar, I could put 3 ball locks in it.
I recommend you buy a used freezer for your first keezer. You learn a ton about keezers, and what youd do differently next time, in the first few months. Plus, all your hardware can be reused in the next keezer! So buy the cheap ass one for your guinea pig.
Go stainless now. You'll never be happier. Forward sealing faucets too.
I use my beer gun all the time. It's one of the best things I've bought. I fill growlers more though with a growler filler, so I guess the choice is yours. Why not both? Lol!
To be quite honest, I love keezers... but bottom freezer refrigerators are possibly the best setup for MacGyver brewers. The ease of use is second to none, and so much less stress on the back. Plus, you can pack away your hops in the bottom freezer!
Wisdom. Lots and lots of wisdom. OP, Listen to @K1ngl1ves