Mechanic says: "I would never rebuild a wheel, even a new one".
I say: "I would never ride a new machine-built wheel without rebuilding it".
Fundamental difference in attitude. I agree that the OP should pony up the extra $30 for a new rim, since he'll need new spokes with his new hub anyway. Then he'll have a spare wheel for essentially $30. But I wouldn't go to that mechanic to get it built. And I wouldn't use an Alex rim, but that's just me. CR-18's are on sale at Niagara Cycle for $19.