It is always best to get a bike that fits, then add an Xtracycle. However, it may be more cost effective to buy an complete bike from Xtracycle. Detail with them, your height and inseam and other fitting issues to see which one of their models would best fit your intended application.
Then when you are ready, add an electric kit of your choice. A decent electric kit producing between 500W to 600W will cost between $450 - $700 depending on your options. AS you can see from my thread here (
http://www.bikeforums.net/showthread.php?t=434383), I am very happy with the outcome
.
Since you are on the rootsradical signup mailing list, you have already seen the custom 2 seaters being made for carrying 2 children out of marine plywood. Some very good solutions there.
Photostream here:
http://www.flickr.com/photos/25872288@N06/
As you child grows, you can add stoker bars for support and mountain bike bar ends for foot pegs. Like this:
