What type of guitar is this? Neck can always be repaired, even if its twisted (not badly twisted). You could re-sand the fretboard and oil it up with bore-oil, get it leveled, re-install a new nut and so on.
"Play with your ears" - Yngwie Malmsteen, Paul Gilbert
Thats what she said...
About the buzz, you have to adjust your neck with the small screw which is on the head.
About the fret deteriorated, you can call upon a guitar maker or a music store but I've ever heard it's so expensive.

If you want to change your neck, you must buy this part (approximately 100-150$ for an occasion) and you may have to call your guitar store to adjust it (I don't the price but it may be 50$...)
mmmm check your neck warp cos that could explain the buzzing at nut. also you could re-fret the neck which is for those that have a set neck, set-thru neck, or neck thru