There's no Nikon equivalent to the Canon "L" designation. There's no simple way of telling whether a Nion lens is from their "professional" range, other than by the price. It used to be possible when they only used "ED" glass in their top lenses, but they use it all over the place now so it's no longer a useful guide.
Regarding the D300/5D issue, a new D300 will cost more than a used 5D Mk I but much less than a new 5D Mk II. Given that the D300 is a crop-sensor and the 5D is full-frame, it's a bit of an apples-and-oranges comparison.