Get the 17 twist , it sounds like you are building a rifle for a specific purpose . Your gunsmith is just trying to steer you down the conservative path, but you know what you want, stick to it.
As to difficulty getting light bullets if nothing else just order some 125 TNT's they shoot very well for an off the shelf bullet. And no it does not need to be 24" there used to be a famous BR gunsmith in NY who no mater what length you ordered your 30 cal barrel it came back 21.5"
Dick