Personally I would tackle improvement of the house first before tackling the solar array. Of those two projects, the sealing attic leaks and insulating is the biggest bang for your buck and would add the most to the comfort of your living areas. Between insulating the current setup or switching the attic to conditioned space, I would much prefer to leave it as is unless you really need the extra storage space. It is much easier and cheaper to insulate is as it is. It is much harder to do any roofing work (say run another vent out the roof) when you have to deal with insulation as well. Having insulation on the underneath side of the roof makes it impossible to find leaks, harder to prevent ice dams (if that is an issue for you) and a few other problems. Just some things to think about.
Insulating basement walls would be low priority for me unless you spend a ton of time in your basement. Also, since cold air is heavier and stays in the basement, I would doubt there would be that much energy savings in your winter heating bill. Insulating would definitely make the basement more comfortable if you spend more time down there. Another reason I don't like insulating and finishing basement walls is because inevitably, things leak, pipes burst and it just creates more damage that must be fixed. Saying that, I have about three fourths of my basement finished. My walls are not insulated either but we don't mind it being a bit colder in the winter. Also, the one-fourth unfinished part is underneath all the pipes that supply the bathrooms and kitchens and has a floor drain so if there is a broken pipe, it won't flood the rest of the basement.
Thanks for the input, the only reason solar is being considered at this phase is that it is something that we plan on doing at some point (we've taken steps to decrease energy use and transition logical items to electric) and the federal tax credit begins to taper at the end of the year.
As for the attic the storage space would be nice, also there is are a couple short runs of duct work that pop into the attic that I would like to bring into the conditioned envelope (I can also envision a scenario where we transition away from heating oil where having the ability to place mechanicals in the attic would be a great benefit) and no matter how hard I try at air sealing I feel that it would be a better air seal on the underside of the roof. Good point on the obscuring any future roof leaks (as an aside our 1991-era roof is likely due for replacement soon-to-this-year). As for ice dams, correct me if I am wrong, but so long as the eave vents are connected the the ridge vent (and not obstructed by the insulation) the melt that results in dams should not take place and the whole roof should be the same temperature (so there will not be a colder spot at the eave to cause freezing of melt water).
As for the basement, as I said it is a blind spot on savings, but the people I know who have done it say they see considerable savings (though that may also be associated with improved air sealing). And then I read things like this
https://web.ornl.gov/sci/buildings/docs/factSheets/basement%20Insulation%20Technology%20fact.pdf , which point to decent cost saving. Our basement has our laundry, a pool table, my brewery, parts of my workshop . . . so basically we do spend a good bit of time down there (often huddled around the wood stove).
Edit: I forget to mention the basement also contains our boiler and it's "waste heat", our heat pump water heater, and most of our forced hot air duct work (yes forces hot air with a cold boiler, not furnace).