1. OS 4.6 supports a 32 GB micro SC card, but those are really expensive.
2. Sound is OK. Won't compare to the iPod as that is made for music.
3. App store works in most countries (UK included) but you need the BB data plan.
4. No OS 6 for the 9000 as it doesn't have the processor or memory. See the yellow sticky at the top of the 9000 section for the latest OS and instructions.
And read this before acquiring any used BB, via eBay or where ever. It has stuff you need to know as buying a used BB is not as simple as buying a used cell phone. Buying a BlackBerry off eBay - BlackBerryFAQ