There are a number of very good gimbal heads on the market. The Nest from Camera Cottage seems a very good value.
The much more expensive Wimberley WH-100 (now WH-200) full size, "J-type" gimbal was the first and all the others are more or less copies of it.
HOWEVER, any of the full size heads essentially replace whatever head is currently on your tripod, turning it into a "long lens only" tripod, for all practical purposes.
IF you have a fairly sturdy tripod and IF you have a reasonably heavy duty ballhead on it with an Arca-Swiss quick release platform, you might instead want to consider a Gimbal Adapter such as the Wimberley Sidekick or the similar Induro GHBA or the smaller/lighter Jobu BWG. All of these work
in conjunction with a ballhead, instead of replacing it. With the adapter in place, the ballhead provides the panning movement while the adapter provides tilt movement. The adapter can quickly and easily be removed any time to use the tripod normally with other lenses that don't require the gimbal.
The gimbal adapters are limited in the amount of weight they can handle (full size gimbals can handle more). But using them with an 80-400mm should be no problem at all. At about 3.5 lb. that's not a particularly heavy lens. I've used a Sidekick for over 15 years with much heavier lenses (up to 500mm, nearly 8 lb.) I often use it with a Canon 100-400mm, which is similar size/weight as the Nikkor 80-400mm. Of the three adapters mentioned, the Jobu is the lightest duty and it might be pushing it a little to use it with an 80-400mm. The other two can easily handle an 80-400.
Here's a web page with pretty good info about the different gimbals available:
http://www.carolinawildphoto.com/gimbal_list.htm Whatever gimbal head you get, you will need to install a slightly over-sized Arca-Swiss plate on the tripod mounting foot of your lens (there also may be a replacement tripod mounting ring available for it, with built-in Arca-Swiss dovetail). This is necessary in order to mount the camera lens to the gimbal. The Arca design mount allows you to slide the rig slightly forward and backward to achieve equilibrium, which is what makes gimbals ideal for use with large lenses... makes it nearly effortless to move them while following a moving subject.
Something else I use with both my gimbals (one Sidekick adapter, one full-size "J-type") is a Leveling Platform. This goes between the head and the tripod, to allow rapid setups. It's often important that the head be reasonably level, and adjusting the legs is sort of a fussy and slow way to achieve that. If you move the tripod frequently, especially on uneven ground, the leveling platform is a really valuable time saver. I would have missed a lot of shots over the years while I was messing with setting up the tripod, if I hadn't been using one. With a leveling platform, it only takes me a couple seconds to re-establish level each time I move the tripod. There are a number of different leveling platforms available. Most add about 1 lb. to the weight of the tripod, but I feel it's well worth it. I recommend the type that can be loosened with a single lever or handle, for quickest possible setup.
You've got more than enough budget for a good, solid setup. And your camera and lens aren't particularly heavy, so you have lots to choose among.