Before you spend your hard earned money, I would suggest buying Chuck Dixon's wonderful book, "The Art of Building the Pennsylvania Longrifle". This inexpensive text is my "go-to" source with every longrifle project I attempt.
I used it a few years back to put together a TOTW "kit" of a fine little .36 cal. limb bacon rifle for under $600. I'm not a craftsman, I'm an old retired school teacher for goodness sakes. You can do it as well, and as Chuck recounts, one of his gun building students accomplished it with the kitchen table as his work bench!
I'd be single AGAIN, if I'd tried that, but this suggests that if you have the time, a modicum of wood-working skills, that building a respectable example of an 18th Century longrifle isn't as daunting as the engraved and carved rifles you might encounter in Shumway's tomes.
A simple, barn gun of the era, without the trappings, will shoot just as accurately, and offer the same sense of history as an exact reproduction of a J. P. Beck rifle might offer at hundreds($$$$) the cost.
As a lefty, I struggled 40 years ago with right-handed TC Hawkens and right-handed Hatfields, but before throwing in the towel, I had a left-handed long gun built as a last resort. I've never looked back, and now hunt exclusively with flintlocks, from doves to deer!