I own a 300mm f/5.6 Caltar HR lens in a Copal 3 shutter that is a nice performer too. The HR stands for Horseman but the lens was actually made by Topcon who used to supply Horseman with lenses. From what I have read earlier Caltars were made by Ilex and Komura.
What's so special about it? I just got back some Fuji 64T E6 from Praus, and yesterday went around with a little light box, a magnifier, and a loupe. The scene is out the company kitchen window, where there is a spotting scope. Using the scope you can see all kinds of things. Looking at the slide with the loupe, it's like looking at things with the scope. Everybody was agog looking at all of the detail. The branches on the Queen Anne hill ridge were distinct, you could nearly see the stars on a flag on the Space Needle, lots of stuff like that. One fellow commented that after seeing that slide, he just didn't understand why people made all the fuss over digital cameras. His thought was, if you could get that kind of detail with film, why bother with digital? Was the color better? No, I replied, the color is worse. Digital just gives results faster.
So you want to give people a sense of wonder about what you do? Shoot some large format E6. That's a real eye-opener.
You don't need an OLD 150.
There are plenty of somewhat recent Multicoated 150mm lenses at reasonable prices.
I have one, and it was less than $200.
BUT make sure that shutter is in good condition.
A shutter repair will add approx $150 + parts to the cost of the lens.
I agree that you need not go past the 1980s for an affordable lens. I have been shooting a Fuji 150 6.3 since 1986. Still sharp, still accurate (Seiko shutter).
To start, stay with plastic film holders, check the slides for light leaks. Clean everything extremely well. While I have used trays, the absolute darkness is always concern. I use System 4 tanks with home made nylon screen tubes and process the sheets just like 135 or 120 film.
Take is slow and work everything through. Write EVERYTHING down and make repeatable systems.
tim in san jose