Before I go too far, I want to tell you that a crank is not compatible with a five through nine speed drivetrain! Obviously as you put more gears on the rear wheel, you need a narrower chain along with different derailleurs that are compatible.

Secondly, if indeed you have a short cage derailleur it is NOT compatible with a 11-30 cassette. The rear derailleur cage is not long enough for that size of cassette, and will hit the bottom of the 30t or 32 t cog. It is made for a 27T cog.

To answer whether this is a low enough gear for you, only you can answer that question. Only you know the ability that you have, and the terrain that you ride. ...

My suggestion here is to visit your LBS, and get answers that are accurate. The information that you are receiving online is very far from the truth and that can lead you into components not being compatible or not giving you what you want. Save yourself some money in the long run, get information that you can use, and go to a shop.