Rebuilding a box is going to cost way more than buying another box, obviously you can't be sure on the condition of the purchased box but it's worth a punt!
Are you getting the crunch from 1st to 2nd or other gears?
Try Redline MTL or GM Synchromesh Transmission Fluid, the GM stuff is meant to 'cure' the crunching and seems to work well for some and reduce the crunching for really bad g'boxes.