From a ground up standpoint there are a few things to consider: You can't redo dirt-side basement insulation so make sure it is where you want to end up. After the already mentioned air sealing focus on putting the insulation dollars where it is most difficult to upgrade in the future. Unfortunately this is basically the entire house which means that every additional dollar should incrementally be put towards every part of the house, an extra inch of insulation here, slightly better windows there, better air sealing tape everywhere. Doing this will allow your house to not have any weak spots that negate the better insulated portions of the house.
Don't forget better insulation and air sealing means a simpler, smaller, and CHEAPER HVAC unit, maybe even a single minisplit will take care of all your heating and cooling needs. A $10,000 investment in insulation may allow you to decrease your HVAC size to a smaller but still conventional unit either ducted or radiant unit netting you an additional $2,000 in further improvements. If you were to spend $20,000 on air sealing and insulation to the super insulated level the HVAC cost drops dramatically because you no longer need an extensive conventional system, that simple minisplit will do just fine netting you a reduction in your HVAC cost of $8,000+ (just guessing), now you have additional square footage because there is no furnace and your basement seems larger without ductwork everywhere. Unfortunately you would then need an ERV/HRV for mechanical ventilation.
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