If your spending that much on a lens, get a Gitzto or RRS Tripod. Carbon Fibre with a Gimbal Head. A gitzo should last you 10+ years or more, same for the head. (and the lens for that matter).
Shooting top glass, off a low quality tripod and head, is, IMHO, 'less than ideal' and potentially wasting money.
I personally use a Gitzo 3541XL with Mongoose 3.6 head for my Canon 800mm Lens. Its a side mount Gimbal head, very light, works a charm.
Regarding Lens, well, exactly what do you shoot in 'nature'. Big animals, birds, somethign in between ?. based on the focal length, I'm assuming not birds.
300mm 2.8 is a spectacular lens, and does work well with T-Con's. I suspect that would be a good choice for what your saying above, and you prob don't 'need' a gimbal with that, (but it will make life MUCH nicer for tracking moving targets like surfers).
I'm not a Sigma fan as a rule BUT, do check out the Sigma 120-300 f2.8, I recently borrowed one for Homer trip on Bald Eagles, and whilst its a bit heavy to hand hold for long periods, results were excellent. with and without t-con's.
For the list you put below, the 300 II f2.8 would be my choice. However, also consider a second hand 500 f4 mk I, which should be available second hand soon as the mk II's hit the shelves. Keep an eye on B&H second hand store.
Hope that helps.
Regards
Mark Farnan