The SM81 is a great mic, but I'd worry about picking one up for less than $150. The SM81 has been around a long time and is used very frequently for live sound. More than a few of them have lived hard lives. I repaired and brought back to life one that had been dropped in the toilet . . . but it still wasn't quite right afterwards . . .
That could be true of any condenser mic, but the SM81 has just been around longer than most of its competitors in the same price range (the C1000 also comes to mind). Caveat emptor, and expect to pay closer to $200 for a used SM81 in good condition.