First you need to know if you camera can take interchangeable focusing screens. I have used them in the past but with my Nikon film camers and not on my digitals. Nikon has the ability to allow you to place grid lines across the focusing area. I use this all the time.
The higher MP camera should give you better enlargements, but you will sacrifice noise at higher ISO. You really need to decide just how large of a print you are going to make. If you are not going into the mega enlargements you just might not need the high MP camera.
I have one but rarely use it. If you are using it for getting proper exposure, you could also use green grass in the area you are trying to expose correctly. It reflects 18% gray. Another method is to use the palm of your hand and open up one stop. Those seem to work well. If you are trying to set the white balance, it would use a white card.
Since you have nikon lenses, I would probably stay with Nikon. Be sure to get a camera that has a AI ring so you will be able to use the old lenses. You can program the focal length and the largest apreture into the camera and will be able to use the lens in apreture priority mode.
I am a Nikon person. This is not to say it is the "best" camera. What I would recommend is that you but your second camera first. What that really means is to not just get the bare essentials camera.
I agree with Jay Pat. The inter-relationships of f-stop, ISO and shutter speed are complex. All three have to be considered in each photograph, based on what you are trying to accomplish. With the D90, just set it on program and see what it does, then start playing with the various setting against the one the camera suggested. Practice make perfect and fortunately with digital you can alway erase and start over.
I would use a 17-35 for the average group shot. Anything wider you might get edge curvature on the final photo. Smaller groups just use a 50.
Don't you also need to know if the camera will accept the card? Some older digital cameras are not compatible with the newer and faster card--I think!!
If I am worried about weight, I just carry my 18-200vr. I know it's slow, but with digital you can always vary the ISO. Fortunately my d300 handles low light situations without much noise. For critical situations I use 70-200 vr f2.8 and my 17-35 f2.8.
Most digital cameras come with the ability to control color through the white balance functions. My most important filter is a circular polorizer which controls reflection off surfaces. Others would be nd filters. I would start with those.
Even though the old rules apply, just remember if you are shooting digital which might uses the smaller chip you should factor in the 1.5 or 1.4 magnification factor of the camera. Hence a 200mm lens is now a 300 mm lens. Therefore a higher shutter speed will be needed.