I could use some advice that touches on depreciation, used market value, and the crystal ball. Want to decide if its a better decision to rent or buy.
I am shooting my first wedding for a relative this summer. Fortunately I'm getting paid. I have a few professional photographers (I've looked at their stuff and its good) who have graciously allowed me to be their caddy for a few weddings this spring. I am confident in most technical areas but could use some more practice in posing couples and group shots.
I currently have a 6D, battery grip, several OCFs, and a decent array of lenses. I am plagued by paranoia and want to shoot the wedding with a 5DIII for the dual card slots and full frame sensor. I guess the higher flash sync speed would also be nice (1/250 as opposed to 1/180 on the 6D) as my YN flashes don't do HSS. I've never had a card go bad before, but there's a first time for everything. I would write RAW to both the CF and SD.
Right now with rebates and discounts it seems as though the 5dIII can be had for $2,000, either via grey market or buying the printer bundle + rebate and then selling the printer. I have no interest in the 5Ds (50 megapixels seems obscene to me and the low ISO is unappealing) and I wouldn't want to spend the $3,500+ that a Mark IV would cost even if it were to get here before summer (unlikely).
I am considering renting a III from Lens Rentals for $200-250ish. But since I don't have it in hand that would mean crunch time to get to know it in a few days. The other option would be to use the Mark III for a few months and either sell it or the 6D but hopefully not at markedly more loss than what a rental would have cost. I've come to enjoy shooting the comparatively low key events that I've been doing over the last year and could see myself shooting several weddings a year, which is why the purchase option is tempting. But that opens up a whole new world with going into business with an LLC, insurance, website, marketing, etc. however its not like I was accosted by a celebrity who thinks my photos are all that--this is a relative who is willing to pay to have me do it. I'm not sure how well I can market myself and if I have the charisma to attract good paying clients. The Canon rebate ends this month and unfortunately most of my assistant-role weddings are in April. I guess part of this dilemma is about self-confidence.
ok... lets hear it _______ 
So you'll never catch me renting a lens and might possibly be clouding my judgement because cameras depreciate far more than lenses. 

