There are essentially three types of plan:
- diagnosis and testing (i.e. doesn't cover treatment)
- outpatient plans (i.e. covers conditions where you can be treated without requiring a hospital stay)
- full benefits plans (i.e. covers everything including cost of a hospital stay)

Pre-existing conditions will be excluded usually on a "2 and 2" moratorium basis, i.e. any pre-existing condition diagnosed or treated in the 2 years before you took out the plan is not covered until you have been clear of that condition for 2 years.
As Des points out above this applies to related conditions.
If you are concerned about a complicated medical history then some insurers will provide a "medically underwritten" policy, i.e. they assess your pre-existing conditions and give you a list of conditions you will not be covered for - but that may be on an in perpetuity basis.


If you have the funds it may be cheaper to "self insure", i.e. park part of your savings as a fund if you need private treatment.