...will it be at this age? am worried that at this milage parts are going to start needing replacement. are merc spares expensive? does anyone have a high milage merc and experience of how these cars run?

If it was made 2000 and up then should be good, on mercedes check for rust around wheels, check maintenance history that may indicate proper maintenance and problems. Last do a car fax with the vin many dealer will do this for you to help sell the car it will show odometer, lemon check, owners and problems.