A brass framed cap and ball (firing) replica of a Colt 1851 navy or 1860 army is great for a first gun. They are inexpensive, will give you experience improving the metal finish, and have plenty of flat, concave and convex surfaces you will have to conquer to do gun work. And best of all you won''t be ruining what may have been a valuable collectors item. Also no FFL is involved with black powder guns (in the USA).
There are replica guns and BB/ pellet guns that are accurate in weight and detail, almost any model you'd want. You can Google Replica guns. I bought a Colt Peacemaker BB gun made by Umarex, fine for engraving.
Hi Jace. I think I will take a little different approach to your question. Curved surfaces are always difficult for beginners so I would consider a semi-auto pistol. My first pistol was a Ruger 1911 stainless. A little tougher on tips but I didn't have to worry about re-finishing it. My first rifle was a single shot falling block again for the primarily flat surfaces. Don't try to do full coverage or inlays. Focus on your design first. If you can't draw it you probably won't be able to cut it. And remember God willing and with perserverence your second gun will be better than your first, and your third better than your second. Plan accordingly.