Considering a basement finish.

We have a 2500 SF two-story built in ’78 w/a full walkout basement. However, for reasons that escape me, the basement is rather archaic in my mind for a home built in that timeframe. It appears the basement was never “intended” to be finished, based on a few factors:

1. Insulation on the ceiling below the main floor
2. No ductwork (basement is unheated and uncooled)
3. Main “drain pipe” going to the septic is 3’ above the basement floor

How would I evaluate the difficulty in extending ductwork to the basement area? How would I know if my existing HVAC (heat pump w/auxiliary heat) can handle it? There is ductwork there that goes upstairs, but none of it “goes into” the basement.

If I want a bathroom there, how do I work around the fact that the main drain is “up” from where a toilet and sink would be?