No simple answer there. If the home was due for some re-siding, that would be the easiest way to get it done.

Old wiring a problem and need to be careful, but should be able to blow in insulation at least in some areas. This requires making some small holes in siding and pumping in insulation material, then patching holes.

Note that, assuming heat is running to achieve a fixed temperature, the decision on various forms of insulation is independent of other things done. There is still a benefit in storm windows or double pane glass even if walls are poorly insulated. Insulting roof or attic floor, if not done already, well worth it even if walls poorly insulated. In the extreme case, it makes just as much sense to insulate one wall even if the other three walls of the space are just plastic sheets. It sounds counter intuitive, but if the furnace is pumping out enough heat to maintain a fixed temperature in the room, the rate of heat loss though any wall surface is independent of the rate of heat loss of the other walls!

Solar is a high up front cost item. Not worth doing if it means sacrificing funds that could improve insulation or replace unsafe wiring.

Difficulty of redoing wiring depends on whether there is an unfinished attic and/or basement. If so, most new wiring can run through unfinished space and then fished up/down wall cavities to specific outlets/lights. Particularly if doing it yourself, electrical upgrades can be done piecemeal. Start with a new main electrical panel replacing the existing fusebox (if applicable). Then as time and money permits, run new romex cable from the panel to existing outlet locations in an area and replace outlets with modern grounded type. Outlets are quite cheap. The real cost is the labor. If you need to hire an electrician, this job is best done all at once. Experienced electricians are good at fishing wire through walls with no/minimal disruption of existing wall surfaces. The biggest problem is if a complete requiring requires bringing the house into current code compliance, since that would require adding lots of new outlets to meet current standards of outlet spacing