there is no easy way in inDesign to combine all those individual files into one PDF to show to the client what the book will look like in facing pages view.
As Willi is suggesting you can export as single pages and set up the PDF to display as facing pages in AcrobatPro (File>Properties>Two-up Continuous (Cover Page)). Not all readers will honor the setting so make sure your client is viewing in Acrobat Reader:
