You remind of an adage we often use when dealing with the “time” issue around fixing processes: “We don’t have time to do it right, but we always have time to do it twice.” This doesn’t mean that I’m not sympathetic to your situation. We are often handed projects and told to just get them done. But I would advise you to put the project through some kind of rigor in order, at least, figure out what you’re in for. It might take an hour or so to set up the initial criteria but after that it takes just 15 minutes to screen the project. Once you do that you can prepare appropriately.
If it’s just an implementation project then you’ll want to to focus on Project Management and build a good Implementation Plan. If it’s just a decision then you can do some cost benefit analyses. If you are concerned that there is no data available, you might ask for leadership’s assistance in verifying where the data might come from. Even if leadership is not flexible on the need for the project – vetting it would give you a prioritized list of where to focus in order to maximize your chance at success.