Technically, it's possible, but I would refrain from doing so. Planet Coaster is requiring a very specific setup for it's custom model files, which it's easy to say, no free models you find anywhere on the internet will have it setup to work with Planet Coaster's model verifier. You're better off learning a 3D program and then giving it a try for yourself. Also you could always ask someone to model for you if they're taking requests.
^ Yes, I would not do it. Most of those models are made for different purposes, and not suitable for games (and least of all modding). Planet Coaster requires very tight guidelines to get models and textures to work properly. It would require just as much, if not more, work to prepare those models for the game.
Thanks for the replies, there was no way I would even attempted to use those [haha] I got confused just from the guides but I guess those website would be handy to use as reference for like you said if anyone is taking requests it would be easier to give an example.
I doubt I'll get any takers for things I want to create (that sound worse then it is)
It's not that hard really. I've done both, a model from scratch and I imported one that I'd downloaded. The only thing I had to do to the downloaded file was add the 6 levels of detail (took 5 mins).. it's definitely possible and easy enough. But like someone said earlier, not all will work.