I have a Mexican Fender Strat with a maple neck that I bought for not very much on eBay three years ago - it had been played a lot and some of the frets have worn and at the nut end of the neck some have dips under some strings.
Can I level and recrown them or would the neck need a re-fret?
You will have to decide whether there will enough 'meat' left to re-crown and polish the frets after leveling - often they are not as bad as they seem.
If you cannot re-crown without the file damaging the fretboard then the frets are too low and need to be replaced.
You can replace individual frets but it might be difficult to fin an exact match so you may end up doing them all anyway.
If the fretboard is made from Maple then re-fretting is MUCH more difficult than say Rosewood or Ebony - probably best left to a pro unless you are willing to give it a go and risk damaging the finish - again that has to be your call.
Measure twice, cut once...