A few points
1) Dont retake a course C or better. Take a different upper level course in Bio or Chem. Retake for MD application doesnt do anything.
2) Depending on the rest of your application, a 3.6 and solid MCAT is competitive. Research that before you drop a boat load of debt and money on a SMP.
3) Unless you have poor prereq grades a formal postbacc wont do much
4) you may be better off getting a few additional upper level courses as "DIY postbacc" to show some solid work
5) spend your money on MCAT prep and do well, will be a much bigger impact on your record.
6) Dont let the 1 C bug you. Really, a single C in a tough class with a sterling record otherwise in a term would appear to me as the "ball-buster" professor
So do all that, apply, and then if you dont get in revisit masters