GreenCard NIW,I140 485 concurr

I am applying GreenCard via NIW, I want to submit I140 concurrent with I485. Can anyone kindly help me with the following questions:

1. what is Certificate of Copies of Original Documents ? Do I need it? if so, How can I get it done?
2. For the employment letter, do I need to submit one? Does it has to be current? I have one dated last year, does it work?
3. Can I get the medical examination done later after the submission?

Thanks for your help.