I can't help you about the BES since I have the internet service, but I can help you on the other questions.
- If your company has uses pop email accounts, then you don't need the BES, but I would doubt that's what you have there.
- You can sync you BB with two different computers. I sync it with my home and work laptops and it works just fine. Your business contacts and calendar will also be copied onto your home computer and vice-versa.
- The OZ Aim is decent. I also have a 8700g from T-Mobile and used it for a while and I liked it. The interface is by far the best I've seen with all the emonicons. The only thing I didn't like about it is that you can only connect to one messaging network at the time. I wanted to be on AIM and MSN so I installed GoogleTalk. GoogleTalk is free and allows you, with a little of work, to connect to just about anything. I'm now on AIM and MSN simultaneously. There are lots of threads about instant messaging you might want to check it out.