Hi All,
Question below, info first.
I’m in an 85 y.o. house 1/4 miles or so from the Pacific (4c) with a slightly sloped 1200 s.f. effectively flat roof. We expect to go another 40-50 years.     Relative humidity is rarely less than 70%.  Roof is at least 40 or 50 y.o. tar over roofing felt, solid sheathing.   I moved in six years ago  and covered a lot with roofing cement as a stopgap.  
The sheathing feels solid, fingers crossed.  So now I’m finally doing a torchdown re-roofing.   
1.  INSULATION:  Assuming the sheathing is not removed, I would like to insulate under the new roof, presumably with rigid foam. Code for rigid foam insulation is 1/2” gap from sheathing, then OSB.     4” to 6” rigid foam on top seems like the minimum to get enough R value to bother with.  
Is this a pipe dream from the cost perspective given a $5k to $10k cost for 1 or two layers of roofing?   It is starting to sound like a major carpentry job, and maybe a more expensive roofer familiar with the rigid foam system retrofit.
2.  ONE OR TWO LAYERS:  I understand I can apply one smooth layer now with a second gritty layer later.  Good idea or bad idea?
3.  ATTIC MOISTURE. I’m confused by all the discussion of damage from attic moisture — does it matter with/without the insulation system?  Roofer plans a couple of vents in the roof – code required I think.   To date there have been two very undersized attic vents.

I’m  sorry this is long but I’d appreciate any advice or comments.