I think your projections for the external options look reasonable. I also think it is reasonable to look at the cost of those options relative to the internal options. A cost-benefit analysis.
Right. The cost is using up money that is available.
Again with the insane over-sabermetric thinking. All wins are NOT created equally, and are not valued equally. For a WS contender looking to gain homefield advantage through the playoffs, 1 extra win could be worth millions. Did we already forget how close the race was between the Cards and Braves this year? Here's a reminder: the Braves lost by 1 game.
A 1 WAR player at league minimum is far less valuable to a WS contender than a 3-4 WAR player at $10M. I am 100% certain you understand the concept of concentrating WAR into single players for championship caliber teams, which this club certainly qualifies.