"because no one would buy preventive care, and insurance companies would pool the sick, elderly, disabled together and have them pay exorbitant fees."
Why would nobody care about preventitive care just because of a free market? Are you concerned about your future health? If you are, is it only because of our current health care system, or is it because you want to be healthy? This argument makes no sense.
In regards to insurance companies charging sick people higher premiums, well they rightfully would. However, costs would look nothing like they do today. Sick people may pay more when compared to healthy people, but that doesn't mean they would be bankrupt. Also, I beleive a free market in health insurance would resemble our current term life insurance policies. In other words, you would buy long policies, say for 30 years, that would be completely independent of your employer. You would purchase these policies before you got sick and they probably would contain only catastrophic coverage, because routine health care visits would be cheaper. We also wouldn't have states dictating what our policies cover, and we could therefor pick plans that more closely resemble our needs. For example, do you want your health care plan to cover a sex change? I don't, but that is built into my premium because of state mandates.