Everything about a cavaquinho setup is judged against a right hand that does not stop. The instrument spends its life under continuous motion, so the question is never "does this note sound clean" but "does this stroke stay clean at tempo".
Action
Low, and lower than feels right if you have come from a guitar. The figures on the batida pages ask the left hand to move constantly while the right hand keeps time, and every extra fraction of clearance is work that accumulates over a whole tune. The limit is buzz under your hardest stroke, not under a gentle test in a quiet shop.
The nut
Slots cut too high make the first three frets sharp and the first position tiring — a classic fault on inexpensive instruments and an easy one to miss, because everything above the fifth fret plays fine. Fret a string at the third fret and look at the gap over the first: it should be barely visible. Slots cut too low buzz on open strings, which is more obvious and less common.
Where buzz actually comes from
In order of likelihood: a high fret, a nut slot cut too deep, action set below what your stroke needs, and only then a genuine neck problem. A cavaquinho is small enough that a single proud fret is the usual culprit, and finding it takes a minute — play every fret on every string, note where it buzzes, and look for the pattern rather than adjusting anything.
Care
A small box with steel tension does not like being dry. It also does not like being left in the sun on a stage or in a car, which is the faster way to lose one. Beyond that the list is short: keep the strings fresh, keep the tuners tight, and check the top behind the bridge every few months for a bulge that was not there before.