If youre worried about stretch: get 3500k and use the uv switch in vegg. 4mm heatsink: like standard 550 clone. 10mm heatsink: it would be 10 with the fins. Dont worry too much about the options. Keep calm, carry on, and grow some weed.
The space and spread in generall: 480s arent ideal, better off with more singles or doubles. You need about 1200-1300 watts for that space. Maybe 6 doubles, slightly dimmed would be better and cheaper as i can not make that space work with less than 4 x 480s and thats a fair bit of over kill but would work aswell. Price out the options.
But as stated here several times: the red+uv boards arent allways ideal, both in price and spread: personally i prefer white boards (in 90 cri if possible but they almost never have) and then add some red and blue/uv strips, that way you can adjust independently and also do end of day treatment with far reds/IR. Also, building part of it yourself, getting spread and framing right for your space gives you a much better sense of accomplishment. Its scary but all of us had to dive in the first time.
If you really care about growing like a pro get High lights, sponsors here on the forum. I can almost guarantee they will do better than any baba board. They beat out hlg, but you might have to get some extra blue for anti stretch in veg.