Insulation: Spray Foam vs. Cellulose vs. New Spray Foam Roof

We have a 1930's brick building we would liket to insulate. It is 8,000 sf and has a low slope roof so the back 2,000 sf cannot be accessed in the attic as it is too short.

Originally the building had a lathe and plaster ceiling, but half of the building has a drop ceiling and the other half has 12" ceiling tiles.

Our options seem to be:

1) Spray Foam
This would involve tearing off the ceiling in the back of the building, spraying the bottom of the roof deck and then rebuilding a drywall ceiling.

2) Cellulose
We might not have to rip off the whole ceiling, instead we could cut holes and blow it in. Then we would have to fix the holes.

3) Spray Foam Roof
Dont mess with the ceilings at all, instead wait until the rubber EPDM roof starts to fail (5-10 years) then re-roof with a closed cell foam roof and elastomeric coating.

I should also state that the top floor has not been renovated and that we need to make this decision before renovating the apartments.

All help is appreciated.