You are the only one that can answer the question of shooting a right handed gun left handed. Personally shooting a right handed gun off the left shoulder does not bother me. As far as flint shards and powder bits ect, a left locked gun could pepper you also.
When I ordered a semi custom smoothbore I opted for a left locked gun. I do not regret it. I thought it would look, well, odd. Even right handed folks do not notice it has the lock on the wrong side.
My latest flinter a 1810 era North Carolina Gillespie longrifle is a right hander. I found that little rifle here on the forum. I guess you would call it a custom since the stock was carved from a blank. It has a Rice swamped barrel, Chambers Classic Ketland lock and fits my time period of interest 1810-1820 Alabama, the time of the Creek War. The maker of this rifle told me " I rushed it but it should make a nice hunting gun.". Its not perfect but are any? When it arrived and I opened the box, Wow !! I just bought a custom long rifle complete for the price of parts.
The left smoothbore is more natural for me to use as far as priming and handling. It took a fine 190lb wild hog @ 45 yards. Its in .54 or 28GA.
The rifle in .45 has yet to make it in the hunting woods. That will be remedied next week. On sighting in it is a little awkward to get the system down as far as loading and priming smoothly as I am working off the left side of the gun and priming over the barrel or behind the cock into the pan. Shoots like a dream after finding out what it likes.
Being mostly left handed I do have the ability to swap sides and aiming eyes and shoot right handed but my natural side is from the left. This is a skill that has served me well in hunting situations. Funny, thing the point of impact will be very close but not exactly the same as the left side.
In closing, I really enjoy my left handed fowler. If I were spending a lot of money in a custom gun or kit I would probably opt for a left handed gun.
I wouldn't rule out a right handed gun if the price was right and it had all the features I wanted and if historical accuracy was my MAIN concern. Left handed guns were made but we're talking about an era when left handed people were considered sinister.
BTW the GPR kit would be very enjoyable as it almost falls together.Final fit and finish is all thats required.
The other kits, TOTW Chambers ect. are parts kits requiring a good deal more work and special tools. You will not save money by building a custom kit like that. Rifle building from a custom kit like that is one of those; It's the journey not the destination things. If you want the finished product save your money and contact a reputable maker. It may take a year and $1500 for a nice plain longrifle or Hawken or if you want to enjoy the "journey" $700 in parts $500 or more in tools, finishing products and about a zillion man hours in labor. A completed semi custom from TVM,ERA, Tip Curtis, North Star West,ect is a good option for around 700-$1200. You can get in the white guns cheaper from these makers. You could also find a nice completed gun like I did.
I in no way meant to discourage you from gun building but some may think you save money by building a rifle,that is not the case IMHO. Its like and old car say a 56 Ford, You will 9 times out of 10 come out better buying a restored car instead of restoring one yourself if the goal is driving a nice old car not working on one.