The estimate of 50 was only intended to be a rough guide for the strength you would need to use decent equipment. Various bonuses can raise the requirements from the base values for that type of equipment, so I don't know what the maximum values are (I suspect 60 is pretty close).
I didn't check the BF merchants very frequently for equipment (relying mostly on dropped or found loot), but it is reasonable they could have the best equipment, with higher requirements than what is available in the main game for that act. At the beginning of act 3 my highest strength requirements were in the low 30s, while at the start of act 4 I had a belt with a strength requirement of 51 and a few pieces of equipment with strength requirements in the mid 40s. Near the end of the game I had a couple pieces of equipment with strength (or agility) requirements of 56 or 57. I don't recall finding very many pieces of armour that I could not use within a level or two, but did have to hang onto a couple weapons for awhile.
