I would generally strongly recommend an older high end body over a new entry level body. New bodies do have better sensors, but because of their lower quality autofocus modules that do not have the possibility of fine tuning, they usually give much lower quality pictures. If you only do landscape or astro photography, and don't mind to focus manually or in live-view, a new body could be a better choice.

Other reasons why I would recommend an older body are that prices of second hand bodies are very low, typically only about 25% of their new price. Many of the extra functions that higher end bodies offer don't seem very important when you are comparing camera's, but actually make a pretty big difference in everyday use. Examples are extra buttons, two command dials end weather sealing. The extra command dial seemed like a very small thing when I bought my D7000, but I ended up using it almost every picture I took. It is very nice to be able to change exposure compensation without having to press a button too.

I switched from a D7000 to a D810 a few months ago, and it is my impression that there is not that much difference in everyday use between a prosumer and a 'professional' camera. When I switched from entry to prosumer the difference was huge though. It was very clear that everything is much better.

About the camera's you mentioned, I don't think the D300 would be a great choice. They are quite old now, and I think you can get a D7x00 for almost the same price. I would not recommend the D5x00 series because they don't have a focus motor, among other things.