Software for editing 3D model

I’d like to carry out some 3D editing (removal of areas etc) from downloaded OBJ files, and my CAD software is really struggling due to the large number of triangles and nodes.

Is there anything available at a reasonable cost? I can’t justify spending a lot for software that I’ll use comparatively rarely!

You could do this in McNeel’s Rhino 6 program. It has a free 90-day trial period. After that you cannot save models but you may be able to still edit them and do screen captures if that helps. If you get traction with it and want to buy it, Nova Edge is currently running a $200 off sale: