i was int he same situation as you about 6 months ago. sencible money saver of expencive dream. I had a choice between a 1.4 diesel 206 03 plate or a new honda civic type-r. big price difference but would have been well worth it. Becasue of the car type the civic would have worked out more expencive for everything, servicing, mot (although it wouldnt be for 3 years), tax, insurance, petrol... literally everything.
With the long commut to work i desided to go for the nice little cheap deisel. i save £70 a month on insurnance, estimated £100 a month on petrol (75miles a day on motorway), the tax is only £50 a year and thats not taking into account maintainance costs.
Work out exactly how much it will cost extra each month, i would also consider a diesel car as they are most echonomical and new ones are just as good (if not better) that petrol.
Hope this helps.