not many music stores that screw up nuts and saddles? maybe we don't hear them on soft very often but i'm pretty sure it happens more often than we think.
My point is: nut and saddle installation, especially on an acoustic guitar, is almost a once-in-a-guitar-lifetime thing. If you can, shell out some money and get a really really good luthier to do it for you. Then you will have no regrets at all.
If you try to save on the money and realise that your nut or saddle was improperly installed, it could shake your morale quite a lot.
Do consider this point before making a decision, and best of luck!