I've got an old MTB converted to an Xtracycle. It is so cool.
It cost about $400 plus shipping for the conversion kit and $50 for the LBS to install it. I also needed a new rear brake, because mine was the wrong kind and wouldn't work with the Xtracycle kit.
So already you're over $500. For $300 more, I'd go with the Townie and get a bike that you know is going to hold up well to the added weight.
I don't know if you can attach a trailer, but I can't imagine that it would be a good thing. Your turning radius would be huge, and it would be too far back to keep track of the kiddos. Maybe get a
Peapod seat from Xtracycle for use when they are older?
The first option--bike + trailer--might work fine, but you'd have fun with an Xtracycle!