Everybody's forgotten Nikons full frame masterpiece the D700
Here's one on ebay that's still in your price range:
https://www.ebay.com/itm/Nikon-D700-MINT-2504-Shutter-Count-12-1-MP-DSLR-AF-Camera/183984881091?epid=1003645913&hash=item2ad65a7dc3:g:txMAAOSwYVpdmgPlThere's heaps on ebay cheaper but this one has
less than 3,000 actuations.
The D700 has a kevlar shutter that is pessimistically
rated to 150,000 actuations.
If you liked your D300, a D700 will put
you in a transcendent state in which there is neither suffering,
desire, nor sense of self, and you will be released from the effects
of karma and the cycle of death and rebirth. :-)
Read the reviews, they are true, I know, I've been driving
a D700 for 7 years, and I'm far from outgrowing it.
Start with a 50mm f1.8AF D or F1.4 AF D lens.
Either of these lenses are an investment.
The sample image below is a shot of a Fujifilm
I took recently with my Nikon D700.
Do the "double download" and check the I.Q.