I am not a doctor, so make of these comments what you will.
As far as I’m aware, no drug has been proven to cure or even slow the progression of PD.
Both Sinemet (levodopa/carbidopa) and Stalevo (levodopa/carbidopa/entacapone) are effective, at least for several years, at reducing the symptoms. Roughly speaking 75mg Stalevo = 100mg Sinemet.
You may wish to consider generic drugs.
Be sure to slowly increase the dose until the symptoms are reduced to an acceptable level.
Until the body gets use to the drugs nausea is common.