Try out a trekking bar first, a used one could be pretty cheap.

A cheap way to convert to drop bars is to use stem-mounted shifters, either buy a donor-bike with them for cheap or look on ebay, etc. for a used drop bar with shifters and brake levers already on it. Some people like flat-bar vintage road bikes (not me!) so you could likely slap your MTB stuff on the donor bike and sell it to break even.

The bars can usually drop right on an MTB with cantilevers... of course I spend some money on SS cables and new housings at this point. I've had good experiences with friction shifters and modern cassettes. If you have 20$ to spare good modern tektro levers are a big upgrade to vintage stuff. Kool-stop pads for the old cantis too.

Gearing on the bike might be great for touring without making any big changes to it, so there are a couple of thoughts on a bare-bones approach I've had some success with.