I wouldn't want to limit the number of changes you can make per week, and it would certainly not be one change per week. Three is probably the very least I would go.
As far as the bench idea, I would do it something like this: starting lineups are 3 F, 2 D, 1 G; bench is 3 F/D, 1 G. Your starting lineup can never exceed 20M; your team can not exceed 40M. That's just a rough idea regarding the roster sizes and cap amounts. But just think of it as something like S44 where you could make changes, but instead of being able to switch in any player, you only have the few players that you selected to choose from.