Best secondhand nikon DSLR for under $600?

I'm somewhat experienced with film photography and i think i should broaden my spectrum to include digital photography.

Depends a bit. The Nikon D50 is old now but compatible with all Nikon autofocus lenses. If you got all the lenses you need you could start hunting for a D90 which should be right in that budget. A used D200 is another outside option.
If you specify what you see as your main focus we could give better answers.

D50? But I got them far more cheaper than that.

The Nikon D80 (or D70) can be found for under $600 easily and is much more robust than the D50. Hey I bought my D200 (better than than all) used for $400 and it works great…