if i may, it isn't so much what questions to ask a potential builder of yer gun, it's examining his track record and that's done mostly via asking around for public opinions of potential builders. for example, take jackie brown. his horribly sordid reputation follows him around whenever his name pops up, and for goodly reason. he is to be avoided like the plague.
for a gun build, you might be interested in the overall gun's design with respect to era/period correctness. there are degrees of "correct build", there are also degrees of what one would *think* such a gun would look like and how it's to be built. not too many 18th century firearms exist and we can always wonder just what we're missing. whatever the design, next up are components selection - stock, barrel, lock, trigger, and all the appropriate furniture.
discussing ALL the parameters of the build with a gunmaker is Extremely important - what goes where and why, embellishments, design accoutrements and carvings and inlays (if desired). lastly is the build time. add all that up in terms of co$t and you may need to sit down to hear the bottom line.
there are are more than a few avenues of pursuit when you've got a gun build in mind. the first is to seek out an already built gun, either new or used. do this mainly by cruising the classifieds in trad muzzleloader forums. another approach is to check in with "commercial" builders such as tennessee valley muzzleloaders, or tenessee valley manufacturing. then there are the Many one-on-one gunmakers - pick up a copy of muzzleloader magazine and view all the ads. and finally, build it or finish it yerself. kit guns can come in a variety of modes of disassembly, based on your ability to put them together. a way to save money is to order out a pre-built "in-the-white" gun that's ready to load and shoot but needs it's wood finished with some stain and clear coat.
ain't all this stuff fun?!