Research past requests both approved and denied. What factors led to a win or less (e.g. clear ROI, compelling story). Confirm that business case aligns with the needs and desires of non-HR decision makers and influencers.
There are a lot of things that are 'nice to have' but what's really needed to move the business forward?
You may feel major sticker shock when you calculate all associated costs, but being transparent in this step is critical