The bottom of my plumbing stack is rusting heavily. Do I need to replace it?
Cast iron rusts, but it rarely becomes a concern due to the thickness of the pipe walls. Those bell fittings are nearly a half inch thick. More likely is that the inside gets choked with corrosion and residue. 

I'd wire brush it, prime it with a high-quality bonding primer, and paint it, merely for cosmetics and cleanliness.

I'm not sure what you mean by "an old connection", but you almost certainly have cast iron pipe from that point to the city drain line or septic system. My previous home had a lot of cast iron and galvanized that was choked down, and after replacing most of it I had the same situation.