Converting yourself depends a lot on what equipment you have and your ability to use it. I would suspect the answer to the original question is that it is cheaper to buy a second hand flintlock, provided it has a good lock on it, than it is to buy a lock and flint breach separately and also cheaper to buy a lock and matching barrel.
Flintlocks are a bit more of a challenge to learn to shoot dependably and well but considering the current cost of caps, flints are becoming rapidly a better choice. I have never had a problem with the wind blowing the powder out of the pan and as long as the rain is not more than a drizzle, rain is only nuisance and forces you to wipe the pan dry before priming. One thing I would avoid is a vent liner with a screw slot in it --- my experience has been that capilliary action seems to wick the rain along the slot and onto the powder charge.
cheers mooncoon