You can do far worse than a Lyman GPR. I've owned three of them, and still own a flinter in .50 caliber. And the advice on starting out with with a percussion model. I started out that way over 45 years ago, with a used T/C Hawken in .50 cal. percussion. It was a great starter rifle, and I still shoot it.
As I got more serious, I eventually graduated to flintlock rifles. I own two custom-made rifles, one a Burks County, and the other a Virginia type. They're both accurate, and pleasant to shoot. Flinters are an acquired taste, but they can be a bit touchy when it comes to optimizing the ignition system and accuracy. However, once they get "locked in", they're tremendously fun to shoot.
I also found a first year T/C Hawken a few years ago at a garage sale. It's bored .45 caliber, and has case hardened brass furniture, something only done during the first year of manufacture. It's a fine rifle, and fun to shoot.
The T/C's are a sturdy bunch. I've taken a T/C 370 gr. Maxi Ball, loaded the rifle up with 200 grains of Goex FF, and fired a couple of rounds with it. Accuracy was fine out to 75 yards, but I wouldn't want to spend a leisurely day at the range with that combination. It will get the job done.
For pure plinking, I bought a T/C rifle that had been barreled with a Green Mountain barrel in .40. You can spend a whole day shooting that, very cheaply, and very easy on the shoulder.
Good luck.
