At an online market place A  Zildjian K Custom Dry Ride 21" inch is offered for 170 Euro's ... about $260, which I think  is a pretty Ok, but it has a 5 mm crack in it. does this crack have a noticable effect on the sound and being not a hard hitter myself, can I expect the crack to get bigger in time? Any thoughts and/or advice is appreciated.

way too much money for a cracked cymbal.  100 euro max for something like that.

Keep your eyes and ears peeled.  There are many millions of used cymbals out there.  And the great thing is, they are still making more every day!

I had a 20" Ziljian ride in the 60's. We were booked 3 to 5 nights a week at that time. I had that cymbal for about 5 years and the crack got to a point where it became a big chunk about 4" long about an inch deep. It didn't sound all that bad until it spread a few more inches. Then it had a weird buzz to it just before the chunk fell out. The guys quit laughing when the lead guitar player took a big gouge out of his guitar neck. That was just before the band broke up.