Hi avus17,
I bought a used 9300 and pondered over pretty much the same questions before I did so you might find my comments of some use.
1. Doctor is correct that the 9500 has a more sophisticated specification but the 9300 is more recent (production stopped in 2000) and most parts are still available. So if you can get a later model - and don't care for 2-way communication - I'd recommend the 9300 simply for the likely longevity.
2. The most common problems seem to be Speaker 2 switch contact problems, flat internal battery, loose glass panels. All are pretty low cost to fix.
3. I'd suggest you get as new as possible - providing it looks like its been looked after.
4. I'm not sure there is much of a preference on the finish. I prefer the flatter finish to polished but that's personal. B&O seemed to have no great consistency in panel and cabinet finish during the model runs.
5. I bought mine via eBay but you have to be really careful. Aside from the obvious misleading descriptions and possible stolen goods, in my opinion there is quite a lot of bid fixing on high value items so you need to avoid getting sucked in. Safer - but probably more expensive - to go to a used B&O dealer. There's several to choose from in the UK - they all pop up on Google.
6. Dealers seem to be pushing good condition models at around £600. I'd say that's fair as these are without doubt classic B&O products.
7. I don't think the 9300 CD mechanism is "improved". It was a newer version but the change was made to standardise components with the Ouverture rather than for performance.
8. I think you can still get most 9300 functional parts from dealers. Some of the true experts on this site can locate items elsewhere too.
If you do decide to buy one, I hope you enjoy it.