Signing in a language you do not speak comes a close second. A sworn translation is a small expense set against what is at stake. Promises made in conversation mean nothing when the written text differs — the contract is what counts.
Rubenmercado
A sensible approach is essentially simple: choose the property first, and let the permit be the second reason. These routes close sometimes at short notice, and an apartment bought only for paperwork can be difficult to let and difficult to sell.
Tax residency is a separate question entirely. Having residency does not automatically make you taxable on worldwide income, and crossing the day-count threshold usually will.
Tax status is a separate question entirely. Owning property does not by itself make you liable for local income tax, and spending enough time in the country often does.
The preliminary agreement typically comes before anything binding: a holding deposit freezes the price for a set period. Check carefully the terms of the deposit if the inspection uncovers a problem.
A rental year is still the reversible decision when you barely know the city. Neighbourhoods change character between seasons, and traffic shows up with time. Twelve months as a tenant is far cheaper than unwinding a bad purchase.