even without the playback button, you can easily access images by keeping the Image Review setting on, so by taking a picture, it pops up and you can then scroll through and look at the others.
As Undertow says, you can keep Image Review on and display images without pressing the Playback button. Be aware that the D300 default setting is for images to be displayed only upon pressing the Playback button. By using the Display mode option in the Playback menu, however, you can set the option to display images automatically after every shot is taken. It seems like this option would solve your problem of not being able to use the Playback button to view your images.
I believe the d300 does not have the histogram overlaid over the image as one of the display options but can be accessed by pressing the centre of the multi selector, is this correct?
There are several options to display the histogram, including separate red, green and blue histograms and an overall brightness level histogram. But, you're correct, for the overlay histogram, you must use the Custom Setting Menu F1 and specify that the overlay histogram appears by pressing the center of the multi-selector button.
If you're able to get a copy of David Busch's book "Nikon D300 Guide to Digital SLR Photography", you can find a lot of this information in Chapter 2 Working With Shooting Data. All of the display screens are shown including all of the information that appears in each display screen, And, after you get the camera, this book wouldn't be bad to have on your bookshelf.
Ellen