"The way that I understand it, if I go the D700 route, I my current lenses would provide an image of comparable size to 35mm film, whereas to get a similar sized image with a D300, I would have to get a wider angle lens, correct?"
This is correct.
Those two lens will work fine on the D700.
I don't see any reason why anyone who is reasonably competent with the cameras you have should have trouble learning to use the D700, provided you don't try to learn everything about it at once. You can start out with the default settings and get very good pictures.