IMHO, get a good condition surplus driveshaft assembly then repack the inner and outer CV joints as necessary if the former is still in good condition. Change the inner & outer rubber boots with OEM for durability & longevity. Now if you can't find a surplus assembly in good assembly, still get a surplus driveshaft with good & complete spline teeth then buy a good quality brand of inner & outer CV joint (ie, Federal Mogul brand is one ex). The inner CV joint rarely gets busted. You can check the "straightness" of the driveshaft by turning it in a lathe machine. Brandnew OEM driveshaft assembly is still the best but if budget does not permit I will still pick a surplus driveshaft over a replacement one. Why? Because OEM driveshaft has its tensile strength tested & surpasses the cyclic stress brought about by its intended tranny & drivetrain as well. By this, a surplus OEM driveshaft is still far better than a brandnew replacement brand. Unless of course you are upgrading into chromoly driveshafts which has a higher tensile strength overall compared to OEM. I can't say much about replacement driveshafts and you can't tell how long they will last or how they perform at high speeds. I dont understand why your mechanic is suggesting you not to buy a surplus since it is used and chooses a brandnew replacement brand over it. This will only make his work easier but leave your car's reliability in question....

BTW, do you mind sharing what the defect your axle shaft has? Maybe it was replaced by the owner with a replacement one earlier?