How would one fix too low action with one piece neck and zero fret?

I don't own this instrument yet but if the price is right I'm wondering how to go about fixing it.

It's a charango (10 string nylon soprano sized instrument) that is made from one piece of wood so no bolt on neck,no truss rod and no standard nut (has zero fret).

Supposedly it has fret buzz due to too low action. I take this to mean too much relief in the neck.

How could this be "fixed" to make it playable?

Would you simply put in a larger zero fret? Raise the bridge by putting more wood under the base? Installing a standard nut?

Pretty much none of those (though I have to guess because you have no measurements).

First, the zero fret won't cause buzzing unless it's smaller than the other frets, and then it would only cause notes on fret 1 to buzz. A taller nut will merely make fretting the lower frets harder and worsen the intonation.

Relief is a slight curvature in the fret plane, and is usually not needed on a short scale instrument. Too much relief means the neck has warped on this one.

You need to know two things:

1. Is the neck flat along the frets. A steel rule on edge, lying on the frets, should tell you if it's close - check this across the width of the neck, along several string paths, rather than in just one place.

2. Distance from top of 12 th fret to underside of strings, measured for several strings. If much under 2.5 mm I'd expect some buzzing.

I'd also check that the top is fairly flat, and hasn't sunk much between bridge and soundhole. If there's a big dip that's probably the main cause, under braced or loose brace, complex surgery might be needed.

If all this is OK, then either some frets are higher than others, or the saddle is too low, or both. These are fairly easy to fix. If the bridge is one piece I'd choose to raise the saddle by gluing wood on at the saddle area and reshaping, rather than removing the bridge and shimming it.

Best I can guess in the absence of pictures and measurements.