Tyre Size & engine ID 90 tdi

Hi I have just bought my first 90 CSW (having owned many 109's &110) very impressed, But it has been fitted with 255/65R16 road tyres, These look like low profile sports tyres on a landy. So I would like to fit some "normal" land rover tyres and would appreciate some advice on size. Also would like to know how to check the difference between 200 and 300tdi and tell if it has had cambelt mod, what type of gearbox fitted as the vehicle is an N reg