Would pop over to the BF Drop bar MTB conversion page first
Show Your Vintage MTB Drop Bar Conversions - Page 295 - Bike Forums
For the wheels, a quick look through the above and the big thing that seem to have been missed, is that the requirement is for 27.5/650B rims with braking surfaces (as you say in the first line for Canti brakes) they don't really exist, as the spec for 650B MTB wheels came along after rim brakes had been replaced for MTB by discs (yes there are a very few available, by these are going to be rim only requiring hubs = build etc and then the cost has to be factored and question why not just buy a 27.5 bike).
For disc brakes, can it be done, yes, but your run into geometry problems, tire width problems, as many 650B tires will be wider than an old 26" frame can take, even with the width your looking at, being as they will be in a different position to 26" wheels, it's a hard question to answer with the very limited info given.
For the rest of your plan, what is the base bike? and how long is the head tube/is the steerer threaded or threadless, as this will you really want a bike with a long head tube, as you will be bringing the bars up and closer to you with drops, if you have a threaded steerer/stem this can compensate for a short head tube, but a threadless you really want a long head tube.
Is a 26" MTB an easy conversion? yes on the surface, but be prepared to spend a lot of time to get it to be comfortable vs a purpose built gravel bike.