I would get the bumper and winch first and let it settle for a bit then you will notice how much of a spacer you will need. If you had a spacer on right away and it sags more then what you added you still won't be level. Just my opinion.
I have the SRC front and when I put it on I noticed some sag. Once I added my three inch lift the sag went away. I am putting my winch on in a few weeks and I am sure I will start to see some sag again. Then I will figure out how big of a spacer I will need.