I assume you're you referring to Step 1, but please correct me if you mean Step 2 CK!

I recommend doing the NBMEs in order (Form 13 -> Form 19) because the curve becomes increasingly harsh. I would take 1 at the start of your dedicated to get a diagnostic. By the "NBMEs" I mean the Comprehensive Basic Science Self Assessment (CBSSA) forms. If by the free 120, you are referring to this set of 117 free questions (http://www.usmle.org/pdfs/step-1/2017samples_step1.pdf), then yes, we do recommend you do this (with the caveat mentioned in my second reply).

UWorld will give you the best sense of what the software will be like on test day, the CBSSAs actually look nothing like the test, but the content is extremely high yield.

Thank you. What's the reasoning behind not doing the 120? I've heard really good things about it from others.

You should do the 117 free questions (link in first post) from 2017, but I recommend not doing earlier editions (which had different numbers of questions). When you use these free 117 questions, be sure to be strict about enforcing time limits and do them on a computer (do not print out, and do not look at the answer key until you are done).