I think the question is how to avoid sinking money into something you end up not riding. I've made several of these conversions over the years, and always end up spending much more than the bike is "worth". But to me this is a hobby, I enjoy the creative process. And all hobbies cost money.

My best advice is to start with the right bike, a nice one that fit and will work with drop bars. Bring a tape measure. Try to find one thats in ridable condition as is. A good place to start is something with 1" fork, cantilever brakes, a 48/38/28 triple and 7 speed cassette. Then go buy a Nitto dirt drop stem, an 80mm is propably what you want. Find some drop bars with short reach and not too much drop. I love the Nitto randonneur. You will be able to sell these at a good price if things dont work out, or keep them for your next build.

Then some brake levers, any drop bar lever will work with cantilevers. Then some shifters, -stem shifters are a cheap way to get it up and running. Then go for a ride!

If you enjoy it: build it up the way you like. Better, lighter, shinier wheels, tyres, levers, gearing, saddle, whatever
If not: remove everything and start looking for a better bike