Hi Jill, i would definately say you should replace the unit. Being 4.5 years old the trouble is if you replace the power supply you dont know what component will fail next. You could end up spening so much money simply repairing the system all the time. It has done well to last 4.5 years but technology constantly changes and computers can very quickly go 'out of date.'
If you already have the peripherals then it probably isnt worth buying a package. Is £400 the budget that you have to stick to?