Zero percent financing or pay upfront

Compare against what the cash would otherwise earn, and read the fee schedule. Genuine zero percent with no fees is cheap money and worth taking; the version with an arrangement fee and a rate that jumps after a year is a loan wearing a costume.

Three to six for a stable income, more for irregular work, and the number that matters is expenses rather than income. The common mistake is sizing it against salary, which inflates the target and delays the point where you stop worrying.