Hi lnorris,
Absolutely. An 8-9" mattress (or in come cases even less) would be fine for you unless there is a specific reason to go thicker because of a particular design that you preferred but it wouldn’t be “necessary” at all. Of course the trick is to put the “right” type of layers (in terms of thickness and ILD) inside the 8-9" that best suits your needs and preferences.
This is strictly personal preference. In an “apples to apples” comparison (the same layer thickness and ILD) the blend in lower ILD’s that are typical in softer comfort layers are likely to be more durable, more pressure relieving, and less costly than the "100% natural material. The all natural though is denser, a little more supportive (which would be more beneficial for a support layer) but primarily … more natural … which for some people is a tradeoff that is worth the higher price. There are lots of people that on principle alone will buy more natural materials regardless of any performance differences.
I would do some serious research on Green Mattress Factory, Dremata, Live for Tomorrow and some of the other names that are now defunct that they have used before considering a purchase from them. They certainly look nice but there is a history there that would make me VERY cautious. I would also read the links in post #6 here.
East Coast organic Mattresses carries some very high quality mattresses but they are also higher priced than other similar mattresses that use the same or similar materials.
I think post #2 here should help.
Phoenix