It means that the fan itself is inside the ducting. So If theses are tubes ( ) (O) the "O" is the fan lol!
You have a little over 4 feet in height plus you going to have to have to keep distance from the lights. I don't have any experience growing in a cab, but you seem to have done your work. You'll want to get a cool tube on your HID light so you can get them as close as possible without burning the plants.
I think low ryder#2 would be a great strain. Once you get some experience with LST and such you'll be able to use other strains.
Good luck!