The first thing I recommend that you do is have a structural engineer evaluate the site. The most important thing you must consider is that you not disturb your piers and more than anything it is the soil which has esecured them for 80 years.

My father converted his basement from a half to full one quite a few years ago. He ended up having to leave 2 feet of soil against the foundation wall, pouring a knee wall and capping that with concrete. The finished space had a 2 foot "shelf" around the inside. He used quite a few feet of this as the base for storage cabinets.

It was also necessary that he install a sump pump for the new space.

No idea about costs. He did his all DIY - yes, a bucket of dirt at a time over about 2 years. A contractor will install a conveyor through one of the windows and have the excavation complete in a few days.

Your new space will be about 20 x 20 at the floor level. Good luck.

My late father dug the basement in the home I grew up in with a bunch of 5 gallon buckets and worked his way up from handheld gardening tools to shovels. It can be done and when it is over it is really awesome to look back on. I take pride in driving by that house and thinking about that and other projects he tackled on that home that many wouldn't even consider by themselves. I say get some professional opinion about doing it and keeping things structurally sound and grab a bucket and spade.