Yeah, forget the trike
At least for the first build. I got a kid's carrier trailer off of craigslist for $25, mainly for the axel, tires and connector. I'll be building a more suited trailer from 1" x 1" square steel tube useing the parts.
Although I saw a three wheeler in Boise craigslist today that was wild. A three wheel mountian bike. The crank had 2 sprockets and derailures with no cross connection between the two back tires with a swingarm shock and spring, had a basket, too!