I'm not familiar with the NX300, but it should work fine that way. On my scanner if I enter the voice frequencies, I hear traffic that way but of course it is not limited to just a single talkgroup. You would hear everything that is coming across the site.
The only NEXEDGE/NXDN showing in the database for that area is Florida Fleet Talk. Now, it is very possible that there is other NXDN stuff out there that isn't in the DB. A good starting point is digitalfrequencysearch.com. That site lists 275 NXDN frequencies in Orange County, but of course many of those are inputs/mobiles and many are likely to be unused or not yet using NXDN.
The last time I was down there, I didn't have the NXDN update on my Uniden, so I can't speak from personal experience.