So a few things, first and foremost, you can find a great condition and still under Rolex 5 year warranty upgraded 6 digit model (216570) with a little time and effort for close to that (low, maybe mid $6's).
The 16570 was produced since like 1990 IIRC, so the model you're getting might be close to 30 years old. Do you know what letter is in the serial number? This will better help know production year of the watch. Is the rehaut engraved? If not, then you know it is at least 12-14 years old.
Example here, it says Rolex all around on the inside of the watch and the serial number is on the inside at 6 o'clock.