Daniel - One of the lensesI recently sold on Ebay was an M42 - 135mm, f3.5. It was a super good lens in almost perfect condition. Not a scratch on it or speck of dust on the glass and it went to someone in Taiwan for $27. He got a steal considering the condition of the lens and case. I had to swallow hard to part with this lens. It was in the best condition of all my screw-mounts.
You will find that if a lot of your photography is outdoors, you will do fine with anf3.5. Originally, the 135mm was a very popular portrait lens in films days and very sharp. With the digital crop factor, it gives over 200mm, a little to much for most portraits. I assume your M135 is very similar to the M42-135 I had. You will like it and the focus is a breeze on them - Bruce