buy tree models from cg trader to place in revit, for rendering in enscape?

I don't think it would be a good idea to place the trees directly into Revit.

Revit will not play well with non-native Revit geometry. Revit does not natively accept OBJ or Max files. Non-native geometry will make your Revit files graphically confusing and heavier than necessary. Depending on the polycount of the geometry the Revit file may crash all together (you also run the risk of corrupting your file). Depending on the format they may not even render with any materials at all. Or the materials may not be properly mapped.

The better option would be to edit them in the Enscape asset editor and properly prepare them for placement and rendering in Enscape.

I made a video on this topic some time ago..