DES - you've given yerself a tough row to hoe with yer allotted gun budget.
the H&R's are light mass weight .45-70's and with the 3 or 4 buff classics i had years ago i just pulled off the butt cap and loaded it up with lead shot to bring the total weight up to 9# or so, and added a kick killer butt pad. all that made it fine to load and shoot with trap door type rounds. the real concern with the H&R's is their break open action - if you want to go really long range, you'll need a tang vernier soule sight and since it's a break open action, that sight and the front sight are on two different moving planes, repeatable sight consistency may become an issue. these will be the same considerations for the henry s/s rifle, which is poorer yet since it's got a real short sporter barrel.
you'd need to find a decent used gun that's dirt cheap, which to me means a pedersoli built gun (under a number of different rebrands, too - avoid ALL the rest of the offshore guns that are not pedersoli built). you'd still need to acquire just decent (not good) sights, as well as all the components to cast good bullets and build good cartridges. to do all this for $900 is a pipe dream and you'd need a good miracle to happen otherwise. LOTS depends on just what you want the gun to do for you, but i sure wish ya good luck!