Deploying software: can I detect if it exists before the install

That shouts for a custom vulnerability (and yes, you *CAN* deploy software with those ... a "patch" is just software after all).

If you're not familiar with those - this will get you started -- How To: Create a Custom Vulnerability Definition in Patch and Compliance Manager .

Aside from Custom Vulnerability Definitions, you can use a Prerequisite query to detect if the app is installed, granted it is not real-time. Another alternative is to use the Detection section, in a package, and that is real-time. Based on your question, I would think Detection would make the most sense since you can detect file version.