Another consideration is process lenses (Apo Artar's, Apo-Nikkor's, Apo-Ronar's, G-Claron's) from the reproduction industry. These were extremely expensive and well corrected lenses that cost a bundle when new, but with the printing world going digital, they are surplus and going for a small fraction of their original cost.

The downside is that they do tend to be slow (max aperature f9 is common) and they are typically barrel mounted (without a shutter). These problems are not insurmountable and many use them, myself included.

More to consider,