I have worn my 4 year old who is 45 lbs for fun around the house in my Ergo amongst many other carriers that he could fit in. I must say that all of my carriers have held up well with no seams coming apart. The Ergo is a very sturdy and well made carrier.
You definitely have at least a year or more until your DD reaches 40 lbs so I think the Ergo will be a great purchase if it works for you, your DH and DD. I know I love mine so much that I just bought another one in a different color.
The Ergo website also says this: "We say our carrier will accommodate you as long as you would like to carry and/or your baby would liked to be carried. The owner, and designer of the ERGO, still carries her son who is now 4-1/2 years old. As you can see from the picture, it will also carry a 90 lb adult."