tergeness
Welcome to the forum!
I had a Pedersoli Sharps 1874 in .45-120, and it will definitely do the job on buffalo! I'm not sure smokeless powder is a good choice in the .45-120, as it will end up with lots of empty case with most smokeless powders. I tried a few loads from my old Ideal manuals, and didn't get good consistent smokeless loads. Accuracy was well within hunting needs though, at around 2.5-3.5" groups at 100 yds.
Velocities with most average loads run in the 1500-1700 range using a 450-500 grain bullet. A lighter bullet in the 350-360 range will give more velocity, but I'd stick with the heavy ones for buffalo. If you use the 405 gr. jacketed bullet, 56.0 grs. of 4895 should give around 1700 fps. If you use the same weight (405) cast bullet, reduce the charge to 50.0 grs 4895, for 1600 fps.
If you decide to use black powder, don't expect to get anything near the 120 grs. that the cartridge used to hold. I found about 105 grs. of ffg would fill the case leaving just enough to slightly compress the charge, with a 500 gr. plain base bullet and a card wad. Velocities with bp were around 1300 with this load.
You'll be glad you got that 32" barrel as it takes that much to burn all the powder the 45-120 uses! Be prepared for a good amount of recoil in that caliber. It can beat you up, even with a heavy weight gun.
Hope this helps!