You are off to a good start. I hope I can give you a bit of direction.

You say the space is 6.5 feet high yet only 4.5 feet of vertical space when the light is hung. Why do you need 2ft to hang the light? Hang it as close to the ceiling as possible. You should be able to do it in about a foot. So, at most you should have 1ft for the pot and 1ft for the light so 4.5 feet of plant space as opposed to 3.5 feet your way. Remember you will have to still leave a small gap between plant and light but with a 400W and ventilation you should be able to leave as little as 6-8 inches.

A 400W will work if you properly ventilate the space. Keep temps around mid 80s.

I like the 5gal pots because they allow for a big root mass, and big roots equals big buds. But since you have a small space you will not be able to fit as many 5gal pots in as 2gal pots. Compromise and go with 3gal pots and fit as many as possible together.

From seed you will have to grow them for about 6-8 weeks before flowering so you will have to top or train them. Once you start with clones just flower when the plants are 12-16 inches and you should be good. Remember that the plant will double or triple in size from the start of flower. Most indoor strains just double.

Hope this helps and good luck.