Ford-8N tractor
低聚 3D 模型

if I bought this could I make it into a mod for Farming Simulator?
and are the wheels a separate model?

Honestly, I'm not hundred percent sure if you can use it as a mod creation for the simulator. Perhaps you can. I have no knowledge what is needed, only that the model you are working with needs to be in low polys for not slowing down the game. I've read that you need Giants editor for creation of mods and it can import FBX files. I know 3Ds Max can export objects in FBX format. And that you need 3ds Max to try to remodel and/or reexport the object, probably with different options enabled, if it shows up errors when importing into Giants editor.
This 8N model consists of about 1000 parts, most of them in low poly. All parts, including wheels and tyres and even many bolts & nuts, are separate objects. I guess you can work with the model to make a FS mod, but I'm pretty sure it requires knowledge and a lot of work to do so.