Well, a total integrated magnitude for a star cluster isn't really very important.
The most important numbers are:
--magnitude of the brightest stars
--number of stars
--Trumpler Classification (see: http://www.astrophoton.com/trumpler_class.htm )
--and, for globulars, the magnitude of the Horizontal Branch, which lets you know how many stars you are likely to resolve

With an 18" dob, I would presume you could see every star cluster in the Milky Way that was not blocked or severely reddened by dust and distance.
And, in good seeing and pristine darkness, stars to about magnitude 17.5. Steve Gottlieb's descriptions are excellent.
In excellent seeing and darkness, you might see a handful of stars in King 2.

In my 12.5", I found King 1 to be an interesting target and like a small cluster filled with very faint stars.
King 2 had a few stars visible, but was mostly a small faint glow.