If you're convinced that the name you've chosen is perfect, then I think you need to either buy the .com or assure yourself that its owner has a valid continuing and non-confusing use.
Though if the name is perfect, there has to be a chance that the .com owner has interests in a similar space?
If the name's just something you like a lot, from what you say I'd not worry about the legal issues, but I'd worry a lot about the diffusion of effort - and I'd probably go find either a .com to use as the primary domain or a .com/.org pair with the .com redirecting.
Either route is going to cost you. But for me, sooner sort things out ahead of launch than risk massive distraction when you're off and running.