x3 on the blown cellulose vs fiberglass batt issue. Even at 1.2-1.5lb open-blow densities the air retardency of cellulose is 10x better than standard density R38s, which keeps it from losing performance to convection between attic air and the entrained air in the fiber layer at the cool temperature extremes. Also, unlike fiberglass cellulose is completely opaque to infra-red radiation, and does a much better job of rejecting the radiated heat under a sun-baked July roof. Fiberglass absorbs the radiated heat at an inch or two of depth, where it's insulated from the convection cooling of the attic air, whereas cellulose absorbs and re-radiates most of the heat at the surface, where it is also cooled to near attic air temps.
No matter what you use for insulation, take the time to air seal every plumbing, flue, and electrical penetrating of the attic floor before adding to the depth. It can be a real PITA, but it's critical to maintaining thermal performance and avoiding moisture issues. Once you're at 2x the R value the attic will be colder in winter, which makes it more likely to retain air-transported moisture leaking in from the space below. In fact, you may be able to spot the more eggregious air-leak areas by moisture staining of the joists or damp rock wool.
Even if you have to special order it, use "borate only, sulfate free" variants of cellulose. Those that use sulfated fire retardents stink and are corrosive of metals should they ever get wet, and every roof will leak somewhere eventually. When that happens it's easy to scoop out the wet insulation and replace it with fresh stuff, but if it's the sulfated goods it can even corrode nails it if stays wet for many weeks or even months. (Box store goods are usually sulfated, but the same manufacturers make "stabilized formula" suitable for damp spraying, which are almost universally sulfate-free.)
Attic mounted air handlers and ducts are universally a bad idea, since it requires making big penetrations in the attic floor/conditioned space boundary, and unless the system is perfectly balanced for supply/returns it drives air-infiltration to something many times the natural infiltration the house would have without an air handler pressurizing/depressurizing rooms relative to one another, making the "great outdoors" the pressure equalizing path for the air to take, which increases the cooling load. If the ducts and air handler aren't well sealed at every seam and joist it makes it even worse! Then there's the issue of conducted gain of the ducts in a 130F attic on a 90F day...
If you have a basement or crawl space where the AC might be installed, that's a much better choice, since it takes a ton or so off the sizing and it'll run more efficiently. If that's not possible, ductless mini-split technology has none of the infiltration and parasitic gain issues, but it could be substantially more expensive if there are many doored-off areas that need separate zone control.
If you really MUST put the ducts and air handlers up there, it might be better (but also more expensive) to insulate at the roof deck with spray foam. To in an Alexandria climate it would take 1-2" of closed cell polyurethane directly against the roof deck, with the rest of the R-value in fiber. Damp-sprayed cellulose (not a DIY project, unlike open-blown dry cellulose) would usually stick well enough to the polyurethane to apply it directly at full rafter depth or beyond. But one could also use a blown-in-mesh solution for the fiber layer (either cellulose, or high-density new-school fiberglass blowing wools such as L77, Spider, or Optima recommended.) See:
http://www.buildingscience.com/documents/reports/rr-1001-moisture-safe-unvented-wood-roof-systems
Alternatively, 1" of closed cell foam followed by a full fill of open cell foam works, and is sometimes cheaper than blown-in-mesh solutions (local-market driven.) The closed cell foam is essential for controlling the moisture loading & drying rate of the roof deck, but more than 2" is both too vapor-tight, and very UN-green, since it's blown with HFC245fa, and would have a much higher lifecycle greenhouse gas cost than the carbon emissions of the energy use it offsets. Open cell foam has none of those issues, with most products using water, not HFCs as the blowing agent. But open cell foam is fairly vapor-permeable (though air-tight), and there can be long term issues with moisture in the roof deck in your climate with an all open-cell solution.
With an unvented insulated roof deck you could then build out the floor for more storage area, which would now be inside of conditioned space. That may or may not be enough rationale for the substantial uptick in cost that an unvented attic solution would carry, but it is WAY more energy efficient to have all of the AC located inside of conditioned space. Insulating and conditioning a crawl space is a lot cheaper than an unvented attic solution, since the R-values required are but a fraction of that in an attic. (The 55-60F deep subsoil temp in your area is a lot easier to insulate against than a 130F or 20F attic.)
