Personaly i rate the Ruger old army somewhere near the top for build quality and if looked after it will out live most of us. The Rogers and spencer is also very nice and it has slightly larger grips so if youve got big hands that might be the one for you. The Remington has a slightly smaller grip but still feels good in the hand and shoots well. Iv tried the colt and didnt like it as much as the others but iv never owned one, some im sure will love theirs and it comes down to personal choice. Best would be to try a few at your new club and see which you prefer.
Dont forget that some Rugers are classed as .44 and some are classed as.45. Both fire a .457" ball and are the exactly the same calibre but if you have a slot for .44 a dealer is not going to sell you a .45. It depends what the dealer has it listed as or a private seller has it on their licence as. You could always try applying for .44/45 and include a letter explaining why.
Iv tried 777 and pyrodex and both are ok once you get a load sorted for your particular gun. Personaly i use 20 grns of TS2 b/p in both my Rugers and 16 grns of swiss fine in my Rogers but dont worry too much about powder yet, Get your gun sorted first. Once your licence is sent off you will have 12 weeks or so to worry about things such as powders, fillers/wads, over grease/lubed wads, size of caps, types of lubes etc. You will get loads of different advice in this area so its only once you have your gun you can start to search for your own load. Have fun.