Ralph, there are two 500mm mirror reflex Nikkor lenses that I know of. The older one is longer than the later, about 15cm long compared to about 11cm long. The first model uses filters in the rear, which must be in place to make the lens work correctly, I think they may be the 39mm filter size that goes in some of their telephoto lenses. I dont know if their later unit requires a rear filter, but probably does.
I myself have a Sigma 600mm f8, wonderful piece of equipment. Focused at infinity it is 12cm long, according to my rule. It uses 22.5mm slip in filters, one of which must be inserted to make the lens work correctly.
I have used the first model Nikkor 500mm f8 mirror lens with a monopod at a motor race track very successfully with a motor drive equipped camera. I couldnt afford the Nikkor, but having hired the Nikkor and liking it, I started looking then came across my Sigma 600mm f8, which was very affordable in comparison.
It is surprisingly light and compact, focusing is very easy and obvious, and exactly the same as the Nikkor I lusted after. Shortly after I picked up my Sigma around 1982, Nikon came out with their later 500 mirror reflex Nikkor lens. I also used that lens and it was closer to the size, shape and working mannerisms of my Sigma.
They are a wonderful way to have great drawing power combined with minimal weight and size. Bang for your buck, they are, in the right conditions, unbeatable.
My Sigma has a very slight blemish on one of the internal mirrors, as a result of that I picked it up for $180 instead of the going rate of around $300 back then. I thought Ill give it a go and see what happens. As it turned out, that blemish is still there, and hasnt enlarged in the 33 plus years I have owned it. The blemish doesnt seem to affect my negatives, including when I have used the lens on a tripod for a bust portrait type of image.
You may find on the outer edge of the main mirror, which is the one you see as you look straight down the barrel, there are slight blemishes. These blemishes do seem to appear on mirror lenses, just like blemishes appear on mirrors in bathrooms and wherever else one has a mirror. Providing they are not too big, mine are about 2-3mm from the edge to where the blemishes end, I think you should be alright.
Look down the barrel and hold the lens at an angle, then focus on the centre of the optic and inspect (within reason) the second main mirror. Turning the lens around in an elliptical manner will let you see/inspect most of that mirror, as well as it can be inspected anyway.
As mentioned, highlights are circular, it is not that big a deal, the in-focus part of ones image is the big deal. I used my Sigma and the hired Nikkors at a time I was shooting almost exclusively colour. Mostly, not always, I would burn the highlights in under the enlarger, I am sure you may have a slightly different way of doing this to any images you may capture, if you pick up a mirror lens.
If the price is right, go for it, they are quite a powerful fun thing to play with.
Mick.