I am planning on putting an air handler and ductwork in my attic to handle heating and cooling to my 2nd floor.  As part of the same project, I am adding insulation throughout the house.  My attic runs the full length of the house and at the peak is about 7' high.  In order to bring it into the envelope, I was thinking of doing the following:
-Flash and fill the floors to airseal (going back and forth about open cell vs. closed cell for this application) and use lots of cellulose to bring up the R value
-Sprayfoam the ducts with closed cell foam (I've read that open cell won't hold up to the vibration)
-Build a box around the air handler with EPS or XPS foam and make sure one side could be opened for access to filter, etc.

Some questions:
1-Is this a good idea and if so, what are the code issues I need to be aware of?
2-Do I foam underneath the ductwork and airhandler or just around and on top of since the point is to allow it to be part of the envelope?
3-What moisture and access issues should I pay special attention to?  I've seen air handlers in attics that have drip pans but if I insulate properly, I don't know if this is still in issue.  Also, if I'm putting all the ductwork under foam, it does make it a little harder to get to in order to repair or make any manual airflow adjustments, I presume.

The cc foam on the ducts needs an ignition barrier to meet code. IIRC 3" + of cellulose would qualify.

In a vented attic with air-permeable cellulose, the underside of the duct is still going to be colder than the outdoor dew point in humid climates, and will experience condensation if it doesn't have air-impermeable insulation on the underside as well. (Whether the cellulose would adequately buffer that depends on the climate and the cooling load, but fully insulating the ducts is more assured.)

I have done this exact thing before. In retrospect, I would have pushed for a foamed in unvented attic. It just performs better. On another note that drip pan is a secondary pan in case the internal condensate pan fails. Most codes will require this for an attic unit.

If there's any way to bring the ducts & air handler fully inside the conditioned space (or go ductless) it makes sealing the attic/conditioned space boundary easier and more reliable, and presents far fewer headaches down the road.

By the time it's said and done it's sometimes cheaper & better to do as geo-dude suggests, make the roof deck the pressure and thermal boundary of the top of the house rather than the attic floor.