Where are you located?

Tossing in a used trans is a gamble, they can be 18 years old now and have issues. I rebuild them and have lots of parts on hand if you are interested.

It's probably a bad synchro or the plastic pads could have come off the shift fork for 1-2. If the trans is pre-1998, the rattling could be from the brass 1-2 synchros they used to use before they switched those to the fibride design as well. They aren't bad transmissions, but 150k miles without ever changing the oil, running it low on oil, or abusing them continuously will wear anything out.