I think is a good deal consider is a D300 and with a lens. Shutter count might be high because the price the guy selling at but I could be wrong.
I see you have a D700 and D7000 already, do you really need another backup?
I can't see both camera will break down at the same time during a job ......
Just like when i do a gig i bring two guitar but i never really need to use a backup .....