please have a look at this post: Specify system variable PYTHON_BIN for Sen2Cor - #7 by Ghanisen
regarding the DEM: Please see the manual at chapter 2.2.7: http://step.esa.int/thirdparties/sen2cor/2.4.0/Sen2Cor_240_Documenation_PDF/S2-PDGS-MPC-L2A-SUM-V2.4.0.pdf
A 90m SRTM is automatically downloaded if available. Currently, no external DEM can be used.