Most European citizens enter visa-free for 90 days. The document that actually matters is a passport with six months of validity.
Citizens of most EU countries, the UK and many others enter Turkey without a visa for stays up to 90 days within 180. Rules change; check your own foreign ministry's page a fortnight before travelling rather than relying on a clinic's website.
Turkey generally requires a passport valid for at least 150 days beyond entry. Airlines enforce this at check-in, and it is the single most common reason patients miss their surgery date.
Nationals who do need a visa can apply online through the official e-Visa portal in minutes. Use only the government site — the commercial lookalikes charge several times the fee for the same document.
Passport, travel insurance, your clinic appointment confirmation, hotel address and return ticket. Border officers occasionally ask the purpose of travel; 'medical treatment' with a clinic address is a straightforward answer.
No. Treatment is possible on a standard tourist entry for the vast majority of patients.