Three things about a sitar have no equivalent on a guitar, and all three are the difference between an instrument that sounds like a sitar and one that does not.
Jawari is the sound
The bridge is wide and its top surface is a shallow curve rather than a point. The string rests along that curve, and the way it lifts off it as it vibrates is what produces the sitar's buzzing, sustaining timbre. Shaping and maintaining that surface is called jawari, and it is skilled work: the difference between a good jawari and a poor one is the difference between the instrument you wanted and one that sounds thin and rattly.
Do not attempt it on a good instrument with sandpaper and hope. It is done with specific tools by people who have been taught, it needs redoing periodically as the surface wears, and finding someone who can do it is a bigger factor in owning a sitar happily than anything you will spend money on.
The frets move, and that is the point
The curved metal frets are tied to the neck with thread rather than fixed into it. They are meant to move: a raga whose frame needs a different position for a swara is accommodated by shifting the fret, which is why the same instrument can be set for one frame and reset for another. Tie them tightly enough not to creep and loosely enough to move, and check them against a reference — the thaat pages show which positions a given frame asks for, and the tuner will tell you when one has drifted.
The body is a dried gourd
It reacts to humidity the way a gourd does, which is to say badly and irreversibly. Dry heated air cracks it; a sudden change of climate cracks it faster. Case, humidifier, and no car boots — the whole maintenance regime is that, and it is cheaper than a single repair to the body.
The order to work in
Strings, then frets, then jawari. A worn main string makes a good bridge sound bad, and a drifted fret makes a correct jawari sound out of tune, so fixing the cheap things first often ends the investigation before it reaches the expensive one.