Welcome to the BBForums.
1. Yes, a Rogers Bold on Vodafone Australia will be fine.
2. These tips pertain to buying any BB used, not just eBay:
Buying a BlackBerry off eBay - BlackBerryFAQ
3. Your browser will work fine when set to the HotSpot browser if you have no data plan and are using WiFi only. You will need a BB Data Plan for full use of email, etc.
4. See PocketMac, and our Mac section here for details.
So check around, do some research and you will be fine.