Travel insurance - check if your travel insurance will cover you for a while. NIB will do renewable yearly insurance for a family of six for $3,500 (with no excess at all) but its limited to one trip out and in.
Pre-existing conditions - the insurers are allowed (and probably will) reject your application if you have any pre-existing conditions. In Australia they would simply not cover that condition but in the USA they will reject you outright. Apparently this is changing in 2014 - https://www.pcip.gov/
HMO vs PPO - HMO seems to have a smaller, more limited, network of doctors outside of which you have no cover at all. PPO allows you to choose your own doctors (within limits) and go to specialists without a referral but you pay some of the cost.