I want a nature camera
First, to answer your lens question, 400mm is unlikely to be adequate. On a digital camera this is going to give only 6x magnification. Some nature subjects will require much more than that.
Also, do not need a fully featured 'pro' camera. These have features which you may not want. Look at lenses first, and let that dictate the camera.
It rather depends on your intended subject matter, but in general for nature photography (I presume you are thinking of vertebrate animals, rather than plants or insects.) you require very long focal length lenses. This is because wild animals are very difficult to approach, and many are comparatively small as well. As an example, you may only be able to get within 30ft of a heron however well you are hidden, and for a bird that size at that distance a 400mm lens will just be big enough. Just.
As a rule you want to fill the frame. So to work out what focal length you need you need to work out the size of the image in the camera. This is not difficult to work out, as the magnification is only the ratio of the subject to lens distance to the (Thoeretical) film/sensor to lens distance. (Most long lenses are physically shorter than their theoretical focal length. That's the true origin of the word 'telephoto', the lens is optically 'telescoped' into a shorter package.)
In reality this varies a little as the lens moves in and out to focus it, but in practice you just use the focal length of the lens. So for out Heron which is about 10,000mm away with a 400mm lens the magnification is 400/10,000 = 4/100 =.04. A heron is about .5m tall (18inches roughly), and 500mm x 0.05 = 20mm. The hieght of a digital sensor is about 16mm, so that's full height, but a heron is a tall bird, so portrait mode might be better, and that will be closer to 24mm.
So in our example, a 400mm lens will do but only for an animal half a meter in size, if you can get thirty feet away. And that's pushing your luck. (The nearest I ever got to a heron without sitting all day in a hide hoping for it to show was twice that distance!)
Most subjects will be smaller, or further away. Getting within 150ft of a deer in clear view is quite a challenge even for an expert stalker. At 1.5m tall with a 400mm lens, the image will be 12mm high. If the subject is a grizzly bear, then I doubt you would want to be that close.
Of course if you are wanting to photograph smaller animals, then the problem is compounded. Especially if they are easily spooked.
In essence you want as long a lens as you can manage, so you can photograph from a comfortable (for the amimal) and safe (grizzly bear) distance. However, as in many instances you won't be able to control that, and the range of animals you want to photograph will vary in size, you really want either more than one lens, or a really good zoom.
Really good zooms of long focal length are very expensive, so two lenses might be a better option, or a long lens with a factory matched multiplier would be almost as good. (Zoom lenses cannot perform at optimum over all the focal lengths available, so really good ones are difficult to design and make.)
So you first need to decide what focal lengths you need.
Then you have to consider camera shake. As a rule of thumb you need an absolute minumum shutter speed of 1/(focal length in mm) for hand-held shots. As you will be using long lenses, with small apertures, you won't be able to take shots hand held.
One (partial) solution is to use an image stabilized or shake reduced system.
Image stabilization is built into the lens, and works by moving optical elements to compensate for vibrations. This makes the lenses much more expensive, and will eat batteries. This has the advantage that it is always optimal for the lens.
Shake reduction moves the sensor in the camera, to achieve the same effect. It makes the camera a little more expensive, but the lenses are a lot cheaper, and that's where most of your money will go!
(Note, that digital image shake compensation is not the same thing, and reduces the image sharpness.)
Of course the traditional solution is a really sturdy tripod. Most tripods are simply not up to the job, so you need to check out as many reviews as you can. But be aware a really good tripod will not be cheap.
The camera mount must be really rigid if the camera is not to move during exposure (A camera with a mirror-up function can help. The mirror is the Major source of vibration in a camera, this allows the mirror to flip well before the shutter fires allowing time for vibration to die away.) and the tripod itself must not flex or twist.
A tripod with the means of suspending a weight underneath is useful, extra weight will make sure the tripod feet are firmly placed and help pre-stress the tripod so any residual 'slack' is taken up. (A simple hook that you can hang a kit-bag on will suffice!)
A good tripod and head could cost £200 or more alone!
As for selecting the lenses....
Canon do some very long focal length lenses but they are also very expensive (£2000+) These include a zoom with image stabilization, and a dedicated multiplier to double the range. A good used example will cost over £1000.
However, you should be aware that Canon are generally quite expensive, and other manufacturers produce similar systems, at various prices. I would look at Nikon, and Pentax, these brands are still well regarded.