Healthcare is available in Ghana but it would be worth factoring the cost of international health insurance into your package, as you may prefer to be evacuated for major health emergencies. There are 2 evacuation services available currently in Ghana, they are WARA and International SOS. For payment of a monthly fee you can attend their clinic, they offer an ambulance service and invaluable advice of where to take a patient for a particular ailment and recommend a specialist, an emergency service out of hours with fully staffed ambulance and they can co-ordinate your medical evacuation if you are in need of treatment overseas. If you have any existing ailments or depend on prescription medication, it would be worth bringing a supply to cover you in case your particular medication isn't readily available. You must carry your current yellow fever certificate on arrival in Ghana as it will be inspected at the airport and you may like to consider the following immunisations:
Malaria is present in Ghana. You can either take a prophylactic or try to avoid being bitten. Minimise your risk of being bitten by having your compound sprayed, closing doors and windows in the late afternoon, wearing long sleeves and anti malaria spray or lotion at night. If you think you have malaria, get it treated as quickly as possible and if in doubt treat for it rather than let the symptoms develop. Take a test and you can find the treatments that you need in clinics or in the chemist. It is a good idea to carry the anti-malarial treatments when you travel back home as they are not always readily available to administer for early treatment.
If you are going to be in Ghana for a number of years then it is worth consulting your doctor regarding the long term risks of the prophylatics available.