I agree with Snake - cept for the diesel comments. The OP does 750 miles a week - that's almost 40,000 a year.
That's serious diesel territory and the fully expensed option does let him get away from it.
The tax on the private fuel is pretty exhorbitant and is also based on CO2 so it's possible worth opting out of private fuel anyway. The break even point on my 320cd for private fuel benefit was about 10,000 personal miles per annum. I reckon keeping it cost me £400 a year, but it was worth it because I hate doing expenses.
He could get a 320d SE with metallic paint and a couple of other bits, but would not be able to stretch to the m-sport or get leather.
320d would be the cheapest option (and most sensible) - I would still go for the 320i msport. 320si is a good call, but they are all sold now - so I think he has no chance.