CANT EXTRUDE CUT AN STL FILE BECAUSE OF ZERO THICKNESS HELP!

I Have an STL file that i purchased online to try to 3d print, I would like to cut part of the model but zero thickness is stopping me.

 I've spent 3/4 evenings on it and its driving me nuts can anyone help? 

forewarning I only really use sheet metal so this is all alien to me. 

I want to remove the contents inside the sketch 

 

surface modelzero thickness.stl