Create a RasterLayer object for UTM values
Create a SpatialPoints object for lat/lon bbox values
Determine the latitude and logitude of the LL of grid
Obtain DEM data projected as long/lat as a RasterLayer object
Generate Extents object in long/lat projection for cropping
Crop DEM data using 'bbox' Extent object in lat/lon projection
Reproject cropped RasterLayer object from lat/lon to UTM