3ird gear grinding - downshift

I am test driving a 1976 MGB. Everything looks great but when down shifting at speed (40-45mph) bad grinding when trying to go into 3ird gear. Does the same even when double-clutching. Will only quitely slip into 3ird at about 20 mph. Down shifting into 2nd at speed is fine and the the transmission other than the 3ird gear downshift grinding is tight and works well.
Is this a big problem? Common problem? What type of repair might be on the horizon? Other than this the car is in very good condition - 118,000 miles and the seller wants $2800. Need I turn and run?
Thanks

Rick, Check out Tom B's post under "transmissions" a couple of inches below this.

The problem down below is falling out of gear with no gas; stretched synchro.Your problem is only slightly different. Still, if it was me, I'd buy a good used unit as they are dirt cheap compared to a rebuilt unit.