i agree the way the doors close on those is so bad, a strip of foam weather stripping won't seal the light in, and an option like chaka said is draping plastic over the front, which now is going to screw up the "hidden cabinet" idea
a way around this might be lining the whole inside with a sheet of panda film, and making a roll-up velcro door inside of the bi-fold doors
as far as exhaust/intake goes you want non-reflective ducting, preferably black, and 2-90 degree bends like an S on both the intake and exhaust, to eliminate further light leaks.
if you aren't using the panda film i suggest caulking all seams with caulk that is flexible and waterproof (latex/silicone)
forgot to mention its great to have 5' of straight run duct on either side of the exhaust fan, to allow it to pull correctly.
i always design my rooms to have the exhaust up high, in the center, with the intake on the very bottom, hooked into a filter/diffuser box.
the box is just so i can swap filters fast, if you wanted you could just use washable filter material and just shove it in the ducting, take it out every week and wash it (you'll be amazed at the shit you get out of it that woulda been on your grow room)
reflective shit isn't as effective dusty, and an intake filter is key to keeping things at their maximum output