Congratulations on your new purchase!
Insulation seems to be on everyone's mind as we try to reduce our energy bills and do something about global climate change. Even though it hurts, I believe higher oil prices have done more than "An Inconvenient Truth" to help us realize our complete dependence on fossil fuels for almost everything. Most people don't realize that the additional benefit of being very energy-efficient in your personal life is gaining the piece of mind that insurance provides: from a pocketbook standpoint, you care much less about what happens to the price of energy. In business, we just call it risk management.
But I digress...
On your ceiling insulation, I will refer you to my anwer to a previous Ask An Expert question, "Weighing energy use reduction and life cycle assessment, which insulation types are best?" That answer will help you take a look at lots of options, including radiant barriers.
One consideration with insulating the ceiling or especially the underside of your roof deck is the triple-layer roof you mentioned. If the sheathing itself is in good shape (no or few weak spots from old leaks) then you are fine. If you think a good portion of the sheathing is going to need to be replaced and the roof is on its last legs, consider replacing the roof before doing major insulation work. If there is no sheathing underneath the original wood shingles, just slats, then I would redo the roof first. Otherwise, you are going to make a mess of and potentially damage your new insulation work.
Your roof is going to get the most heat pounding down on it, so you want to focus most on the attic. However, your thermal envelope is like an ice chest, and if the sides have no insulation, the beer doesn't stay cool. You didn't mention what all the walls are made of, but with your shiplap walls, you might be able to take off the top row and blow insulation into the walls. Both cellulose and foam would work. Cellulose is more environmentally friendly and foam will do a better job of filling the cavity all the way up to the ceiling line.
Depending upon the exterior finish and the difficulty of filling all the walls cavities, "outsulation" might also be a viable solution. You can sheath the exterior with foam board and then stucco over it. Figuring out all the detailing around windows, doors, foundations and architectural features is definitely a chore.
A radiant paint will help as well. Most ceramic-bead radiant paints are elastomeric and made for roofs, but they can be applied to exterior walls, as well. I put it on both the roof and walls of a 1970s addition to my 1930s home and it performed extremely well.
One final consideration is shading (of which radiant paint is actually an example). If your home has deep overhangs or even 360-degree porches, you are lucky. If not, consider vertical shading (trees, shade walls, trellises with vines) on the east and west sides. On the south side, use horizontal shading such that the winter sun can warm the house but the summer sun is blocked from directly reaching the walls and windows. This won't make your ice chest more insulated, but it is equivalent to putting the beer cooler under a shade tree rather than out in the middle of the yard.