Does anybody know what the stock offset is on 2017 c300 coupe base? What's the max offset I can do before needing spacers (trying to avoid that)?

 

https://www.wheel-size.com/size/mercedes/c-class/2017/

You should only need spacers if you are trying to achieve an offset that is not available from a wheel manufacturer, or to achieve a different offset from a set of wheels you already have, or to gain clearance between the wheel spokes and brake calipers.