There are a number of issues that you need to consider, and each of your needs, wants and desires are likely to be vastly different to others. Take the time to list these needs and wants, and then priortise them so you have a clear basis for any decision that you eventually take regarding your photographic purchases; it can become a very deep money-pit AND provide many disappointments if not undertaken in a clear and methodical manner. There will ALWAYS be something better, faster, sharper or more appropriate, unless you have both an unlimited budget and a truck to transport the equipment.
As a budding wildlife shooter for personal memories only, I am happy with the D7000 that I use, but budget constraints forced me down the ladder when choosing lenses. I us the Nikon 70- 300 VR as my prime wildlife lens. Sure, I would love to have the 70 - 200 f2.8 and a converter, (or better still, a 300mm f2.
, but the price of that lens combination alone (new) is over R30k. I chose the D7000 primarily for the manual controls I have. Similar controls are available on the D5100, but many of them are buried within the menu system, something that was a deal-breaker for me.
While the D300 is a great camera, and built like a tank with all the bells and whistles, it was released over 6 years ago, and sensor technology has moved substantially since then. I believe that, bang for your buck, you would be better off looking at the D5100, unless you could justify moving to a D7100/D7000/D5200 (new or 2nd hand). Also, while not a long lens, do not discount the quality of the 55-200 VR (or 55-300 VR) kit lens, which, together with your selected camera, you may be able to get within your budget. The 70-300VR is, however, a better option, as it is more robust and optically a little better.
I have tried to keep the discussion around your stated budget level, and while there may be 2nd hand options within your budget for better lenses, you generally pay for what you get. For example, a new 70-200 f2.8 VRII (great lens) is just short of R30,000 and a new 200-400 f4 is currently over R90,000.... The 800mm VR f5.6 is around R180,000, should you want to go for the big stuff.... (dreams are good friends)
Finally, for your video needs, I would suggest that you look for a video camera, rather than try and use your DSLR for that mode of photography. DLSR's are designed for stills photography and video cameras are designed for video. A porsche/ferrari/bently CAN transport cement to a building site, but it is not the most appropriate tool for the job). Feel free to send me a PM, should you believe there is benefit in further discussion.