For general use including hunting I would highly recommend a 50 cal. It just gives you more options , period. You've stated that caps are no option. Then also 777 is no option. Yes, you can do a 10/90 , 20/80 split or what ever but why? If you have the ability to buy real black powder then get the 3f (preferably) or 2f you need to work with a flint lock. Just because you have a good supply of 777 doesn't mean it's the correct answer. I wouldn't use 50W motor oil in an engine designed for 15W.
As for a recommended brand, based on availability, price and quality my choices would be: 1. A good used Thompson Center Hawken or Renegade, again in 50. 2. A good used Lyman/Investarms Great Plains Rifle / Gemmer (or Hunter model for slugs). You will find them advertised here regularly. Only the Investarms are avail new.
One last thought, if you are determined to use up your 777 you might consider getting a caplock in the future. Good luck and have fun!