Most of your questions can be answered on their website:
Unication USA
As far as zones - you can have any number of profiles on your PC and each profile can handle more zones than you'll ever need. You can only load 1 profile to the radio at any given time. I will note that I have "overwhelmed" the G4 with too much programming in a single profile. It can get ugly when you do this (you may want to throw it against the wall if/when this happens).
I haven't seen issues regarding "time" limit of recordings but I do routinely see the 1000 recording limit hit. When this happens, recordings that you haven't "locked" are overwritten (circular buffer). If you monitor alot of stuff, the overwrites can happen in a matter of hours since each transmission is it's own recording.
The G4 does a great job on Phase 1 simulcast systems in your area - Baltimore City, Baltimore County, CMARC, etc. Of course, it remains to be seen how well the Phase 2 capability (hopefully soon?) will work. Keep in mind that there will be an upgrade charge for the Phase 2 capability when that comes --