i am using cxa3070 ,, I have 2 of them in my 2 x 3 cab flowring, its doing so very well too, mine are 3000k cobs, and you can put together a cxa3070 for around 70 dollars now,, for all the parts,
i have my cobs 10 inches over the tops, with no problems after 2 days at that height, I have a mars hydro light with differant colors , but I cant run it because the temps get to high, so I am taking it out of the box today so the cobs can spread thier light better.
your idea of putting some cobs in and then adding more later is a good idea,, its exactly what I am doing at the moment, I will try and have 4 cobs in my cab for the next run, if not 6 total, just money prevents me from doing it all at once, seems like 4000k is a good middle of the road spectrum, but My plants were vegging just fine under the 3k's