We love our 2500 Burb. It is designed to do exactly what your family wants. I looked for about a year (I was being picky on colors, equipment, etc) used 2004-2006 3/4 ton Burb/Yukon XL with the 8.1L. Used 2008+ were just as difficult to find and not worth the discounts over a new one. So, we bought a leftover 2011 in Oct 2011 when 2012's were on the lots.
Used 1-ton vans are far less $, like half for same year/condition. They just come with less equipment "loaded" than the base LS Suburban and are only 2WD. If that's ok with you, they make great family TVs.
If you are over GCWR by 100 pounds, you are probably over the receiver rating at 900-ish pounds and payload by about 500 pounds.