If it were me and because everything is so old, I'd be really tempted to just replace the existing components with similar, or slightly better, specs. You could buy everything and install yourself and there'd be no point in pulling permits. Efficiency on everything is probably down by 20% due to the age, so just getting that efficiency back is worthwhile, imo. And if you wanted to add a little, you could try to find juuuust the right panel with the same voltage and like 10% more current, but in the same form factor (because energy density has improved a lot in the last 20 years) without having to rewire everything. Things get expensive beyond that, like starting over, which means you do it with a phone call, which means $$$.