Hmmm, much depends on your budget. If you are looking for a used bike, see if you can find a reasonable steel bike that already had drop bars as the frame has specs more designed for that.

That being said, I did convert my mtb, so here are the key things required.

Drop bars
MTBs have a different bar diameter and use different stem angles and lengths. When I swapped mine, I also measured out what stem length I needed so the hoods would be about the same distance as my original flat bars from the seat. I also went with a stem angle that was the same as the original so I was not way too low when in the drops.

The stem I used is a standard quill type stem, but I had to find one that accepted a 26mm clamp as that was the type of drop bar I am using. I should note that 26mm or 25.4mm drop bars are getting harder to find. They also make conversion stems so you can use current stem and handlebar set ups.

Brakes
This is a biggy. Older MTBs will have cantilever brakes and the brake levers will not fit on a drop bar. I bought a pair of Tektro brake levers which are designed to work with direct pull cantilevers. I am not sure if there is a brake/shifter combo that will work.

Shifters
My mtb had separate, thumb shifters so I fabricated a custom mounting arrangement so I could keep using them. Most conversions swap the original mtb shifters for bar end shifters.

If you want drop bars for hand positions there is also some trekking bars that would give you that and allow you to keep your original shift and brake levers.

For me, I love tinkering, I have had the mtb since new and was the most cost effective and fun way for me to get a fun, comfortable, enjoyable riding commuter.

Have fun and good luck.