No comment on the feasibility, but here are the numbers (I think) to calculate the size of your balloons:
Air weighs about 1.25 kg/cu m
Helium weighs about 4 g/mole, or 4 grams per 22.4L (assuming ideal gas properties)
at 1000L per cu m this converts to about 0.18 kg/cu m.
Hydrogen weighs about 1 g/mole, or 1 gram per 22.4L
at 1000L per cu m this converts to about 0.045 kg/cu m
So:
Helium will "lift" about 1.25 - 0.18 or 1.07 kg for each cubic meter of volume
Hydrogen will "lift" about 1.25 - 0.045 or 1.2 kg for each cubic meter of volume
If hydrogen is diatomic (H2) the the hydrogen number becomes 1.16kg of lift per cu m.
Somebody please do a sanity check on these numbers and let me know if they're all wet.
Have Fun,
-Dan M