First, caulk every seam you can, make the exterior wall sheathing as air-tight as possible. Caulk the bottom plate to the subfloor while you're at it.
Then, put 7/16" OSB on the interior of the studs, marking where the studs are. Drill a 2-1/4" hole about a foot from the bottom, and about a foot from the top of each stud-bay, and use a rental-blower from box store to blow cellulose into the bays. To do this half-right, stuff a rag in the top hole to keep the blow-back under control, and start at the bottom hole, angling the nozzle first toward one side, then the other. (Use a rag or a cut-off sweatshirt sleeve over the nozzle to control blow-back at the blowing hole.) Blow from the bottom until the blower stalls, then go to the top hole and blow toward the upper corners, then down until the blower stalls, and you can't pack any more in there.
On the ceiling you'll need to fit OSB on the under side of the 2x4 purlins first, caulking the seams as you go, then put some OSB under the rafters and treat it in the same way that you did your walls. You may have to drill/cut vent holes to the exterior at the overhangs/eaves to let the purlin bays breathe or even a small amount of drip-drip could compromize your outer OSB layer. It's probably worth running 1x furring at right angles to the purlins first, mounting the OSB to the furring, that way the roof would be fully cross-ventilated for maximum drying capacity.
To calculate the amount of cellulose you'd need to buy, figure on hitting ~2.5lbs per cubic foot with this method. In 2x6 cavities that would be about 1.1lbs per square foot of surface- buy 10-15% extra just in case you get lucky and get a blower with better seals and a higher horsepower- box stores will usually take unopened bags back. So for 5-sides of 64 square feet (assuming 2x6 rafters) you're looking at 320lbs- so buy 350lbs. From distributors cellulose comes in bags of everywhere from 20lbs to 40lbs, but usually 20-25lbs at home-centers. A quick search on the orange box store site looks like it's running about 50cents/lb through them, and most usually throw in 1-2 days blower rental for free if you buy $100 worth of goods or more.
In a 2x6 framed structure, center-cavity you'd be at ~ R18-20, and over R15 after factoring in the thermal bridging of the framing, assuming 48" on center. Blown insulation leaves no gaps or voids, and will outperform ANY batt insulation in the real world. If you want more, you can put up fire-rated foil-faced iso on the interior at about R6/inch. Whether that's cost effective depends on the cost of fuel and how well insulated the boiler is.
The $64,000 question is, what's under the floor, and how to best insulate that, which may depend on how you did the foundation for this thing.