sounds reasonable to try and jus change the expansion tank at first. how many miles have you done? I had a gradual coolant leak, never really overheated jus needed topped up every week or so and i 1st had the expansion tank replaced, but turned out my radiator matrix was all swollen and past its best, hence had that changed as well.
If you do get the radiator changed, it worthwhile changing the water pump, thermostat and drive/accesory belt also.
Found this DIY of sorts over at E46Fanatics: