OK, so you have 3 basic problems - drafts, venting and insulation. Drafts are simple to fix but sometime time consuming. Obvious areas are windows and doors that aren't sealed properly. All the insulation in the world won't help if the wind is whipping through the living space. For the attic, you should add soffit vents before insulation. That way, you can put little baffles in front of the vents and you'll avoid spaying insulation over them. You should also have roof venting. In summer, the air will come in through the soffits and out through the roof vents. For insulation, I would spend a little prep time before you start spraying cellulose. A few cans of expansion foam will do wonders to minimize air flow from the living space to the attic. You will probably have to roll back the existing insulation to see the problem areas, but you can roll it right back when you are done. Then, you can spray in cellulose over the top.

You do probably need a vapor barrier at least on the roof, but I don't know for sure. Those are typically plastic film or metallized film bubble wrap that is put on before the sheet metal. You might be able to staple plastic film from inside prior to insulating. I'd recommend talking to a local metal buildings dealer and get their advice for your local conditions.

"Get references for an energy rater and make sure that they are trained." ditto I am reading in other forums of 'free' or minimal cost energy audits. if the sole purpose of the audit is to sell the fix up package and leakage is defined simply as equivelent to 3x5 hole in wall it isn't showing you where the leakage is located. Ideally have the audit done by an independent company that tests before and shows you leakage sites with recomendations as to how to seal them..and tests once sealing has been completed. This way the auditors loyality is to the consumer..not the company doing the repairs. caulking seals a lot of leakage and is flexible enough to provide a long lasting seal. mastics for ductwork and hvac related leakage will provide a long lasting seal for hvac. knowing where the leaks are and how to seal them is the true goal of an energy audit..imo & experience. foam insulation provides air sealing and insulation in one. sealing the leakage and conventional insulation is the equal if enough of the air sealing is done. this takes time and materials.. first you want an air barrier..then insulation. recessed lights that are not ICAT (insulation contact air tight) oversized cuts at bath fans, stove vents are large leakage sites. cuts in ceiling where supply boxes are installed should be mastic sealed as should return air and all duct connections,and plenum connections. sealing these (and other leakage sites) will improve your air barrier to the attic.. if you install cellulose keep in mind that the 'dust' from the newspaper & borates is very fine and can enter the living space and duct system. this is recirculated throughour the house. any gap from attic to living space will allow these particles to enter the house, as will leaks at supply boxes, r/a and plenums. one place to find energy rates is natresnet.org best of luck.

I would go with the 1.5" insul board and forget the strapping. It is more important to have a continuos run of insulation than the extra thickness and with the strapping every 24", there is NO insul board covering the wood at that point. Just mho.