IMOH Entourage is absolute pants and have not been able to use it, but Apple Mail which comes with your Mac, again IMOH is great.
Ok it doesn't have the calendar in it, but easily integrates to your iCal same with your contacts.
If anything I would recommend you use Apple Mail.
As for Office. I haven't been a big fan of open office, but like the concept, and have used it once or twice.
I currently use Office on all our Macs, and short of a few hicups, which aren't serious to the operation of office (just individuals preferences) its been great for us.
iWorks, yes its much cheaper and will do what you need it to do, I have it, but haven't looked at it since putting office on.
Its really down to your own preference and if you have money to spend on it. If you are looking to cut back - open office, if you are looking to be Mac loyal, stick with iWorks and if you know how to use office and like it, spend that little bit more and go for that.
The only caution I would throw you - if you use macros on any of your documents, Office for Mac doesn't support this, so all your fancy spreadsheets if any wouldn't be any use on the mac without re scripting them.