A tripod that isn't cheap/broken like mine currently is
I'd esp. advise a good tripod *head* that allows for smooth movement even if the camera is held stable - in my case that was more expensive than the actual tripod. And you probably should get some advice on what system to use if you didn't decided it yet (I'm using Arca Swiss, but it's rather uncommon in Germany). As for the tripod legs, do you need max. stability for extra long exposures or just a sturdy version that you still can carry yourself w/o a golf cart?
The real question here is which things I should get now and which can wait until summer. Possible combinations...
I cannot give you any advice here, but one reason for the Canon gear is that the 5d3 has a similar usability to your 7d (which you still might want to use for the 1.6x crop?) and not having to re-think once switching cameras should be a plus for a pro photog.
Also a new 14-24L is rumored to be announced any day :-o so if you've got the $2500+ it'll probably cost that might be a good upgrade over the 16-35L.
A tripod that isn't
that's how I got my 17-40mm and it was $150 less than ebay and in superb condition
You Americans should really appreciate having a working craiglist btw, in Germany there are many smaller different sites but nowhere photog gear is to be found, that's why I just bought a *new* 17-40L (with Canon rebate) because the ebay prices were 85+% of the new lens...
Horse photography is tough business.
But seems to pay, so watch out for envious photogs crossing the street :-p