The estate is not as common as the hatch but there are still plenty about from both dealers and private sellers. Although the Focus is a reliable car, a full service history is good to see - pre-facelifted models will be cheaper but avoid the Studio unless you're on a strict budget. Check for interior wear and tear if it's been used as a workhorse.
Small estates are not as popular as larger models (like the Mondeo for instance) but they are a good choice for family buyers who don't necessarily want a people carrier. The diesels will be the most favoured - they cope better with full loads and offer impressive economy.