I have a design which will cut the prokaryotic vector about 30bp away from the ampR promoter. Will this cause problems or should I look for one further away?
It is impossible to answer your question with the details you provided. A promoter region spans about 25-35 bps. So when you are counting 30bp away it depends upon where you are counting from. 30bp away from the most upstream element is fine, 30 bp away from the most downstream element (or the mRNA start point) is probably not fine. However the promoter region should be well characterized and with a bit of literature searching you can determine whether that site is within an essential region or not.