First off, if you feel you are in fact competent, secure a letter from a qualified physician (preferably a Psychiatrist or Psychologist w/experience in PTSD) that would be willing to write you a letter to present to the VA in support of denying the proposal. My experience has been, that you will be declared "competent" w/a solid and well crafted letter. Testimonials from acquaintances to support the doctor letter would help as well. Since the opinion came from VAMC doctor or a doctor contracted w/the VA, you may be better served trying to get the opinion outside the VA/VAMC realm, if possible.
If you are agreeable to the incompetency, then you would likely be set w/the VA's offer of the SDP program. In effect, a VA fiduciary would be controlling your compensation and how it is spent. This is not agreeable to most of the vets I served and often we try and get a trusted family member to act as the VA appointed fiduciary. This is assuming that the vet either doesn't want to or can't get a letter from a doctor to support competency. You, for now, may truly be incompetent to handle your finances and it may suit you better to have a family member w/a pretty clean legal track record, to be the fiduciary.
Unfortunately, it will be a while before the VA releases the compensation. If you get on it right away, I would suspect 2-3 months depending on the VA regional office and how quick they work. I would also ask, are you under rated? Are you working? Maybe you should be at the 100% rate permanent and total. That would be the goal here, if appropriate.
Good luck and PM me if desired.