To buy or not to buy transmission

A guy in my area is selling a rebuilt 2000 transmission (no torque convertor) for a Rodeo for $300. Its still on the shipping skid.

I've been having a whine coming from my transmission (2001 rodeo sport 4x2, 161K miles) when, at a stop, I'm in Drive. I'm fairly sure that its coming from the transmission because I put a stethoscope to it and I'm sure the fluid is at proper level (checked it a week ago). I changed the filter and partial fluid in december (4k miles ago). I'm thinking it would be pretty cheap insurance to purchase as I've heard horror stories. I've only had it for about a year and don't know the maintenence history.

Thoughts?

If your transmission is bad, you're going to want to replace the torque converter. If it were me, and the noise isn't waking up the neighbors, I'd wait until symptoms get worse. The noise could be nothing.