Help a feller out
my opinion would be that toe-nails would be sufficient since the purpose of the ceiling joists is to provide support for the ceiling, not as a part of the roof construction. the person who's opinion matters is the building officials so you may want to run it past them. The BO may require an engineer's evaluation to ensure they is no compromise of the existing structure since it was not designed this way originally.
"You get what you pay for, and sometimes free costs more!"