I do not recommend a first generation for shooting. If you look for a good shooter, buy a thirt generation in .45 colt.
The differende between a blackpowder frame and a Nitro frame is. The Blackpowder frame has a screw in front of the frame to secure the cilinderax. The Nitro has a crosbold true the frame. In first generation Colt SAA this do not mean that it is fit for nitro. Only the one made after 1900 are fit for nitro.
Hope this helps