Used Tremec

hey guys Im looking at picking up a used tremec, because my stock T-5 vibrates in 5th pretty bad going down the highway @ 70. The guy says its a 3350, but i cant find anything on a 3350 anywhere, so Im assuming its just a type o and he really meant 3550. I just wanted to know what all i should ask when looking at buying a used transmission. I know i should ask about mileage, missed shifts, and if it was a race or DD car.

The guy is asking $750 for the trans, BH (appears to be stock), pro 5.0 shifter, and McCloud clutch, does that seem like a good deal assuming its in pretty good shape?

yeah im sure its my tranny, because the vibration is only in 5th gear starting at around 70. I can run it up to about 110 in 4th and have no vibrations, so i know its not the DS. There is also super rough vibrations when i go form 4th to 5th at anything resembling highway speeds. I have recently changed my U Joints and im using an energy suspension mount. My wheel bearings are less than 30k old. Im almost sure its comming from the transmission, the thing has more than 200k on it and it had a U Joint let go at 110, has had a tab break off and was welded back on.... it deserves to go..

I assume there is on way i would be able to check the syncros without the trans in a car?

I am going to check out the transmission this weekend and hopefully pick it up.