I dont know loads but check anything electric as it's a Renault, check the glow plugs don't take ages to fire (there should a a glow plug light on the dashboard that comes on when you turn it on before starting, check it runs OK doesn't smoke too much, is it clean or battered? A good diesel can go on, listen for any knocks when you drive it, check the tyres, I usually think if a car looks well looked after then that's usually a better sign, if you have a link I might be able to tell you a little more?