[Pathfinder] So I'm adding tree parts to a bunch of animals and need some help making it work

So in my campaign I have my PC's going with a caravan that is trying to take down a large trading company, so while they travel from town to town I like to add some stuff in between to make things more interesting then saying "Yeah you make it to the next town"

So one of the encounters that i have planed is they will stumble upon a Dryad that is far from her guardian tree, she had been away from it for a while. The Dryad will try to tell them to save someone and tell them witch direction they will go in. While they travel deeper into the woods they will encounter Wolves and forest creatures, but part of their limbs will be made of tress and wood instead of flesh. They lost their original limbs to some accident so they where given these wood limbs by someone through the means of magic. The guy who did this is in trouble and that is who the Dryad is trying to save.

So I just want to run by if I should add the construct subtype to each of the creatures because they where kinda of made with magic or should I just add the weakness vulnerable to fire to each of the wood creatures. The levels of the PC's are level 4 so I don't want to make it to hard, just make it different than just fight some wolves on the side of the road.