I have done this recently too. Frankly I would choose it not on the emissions/tax bracket as generally you will get a bunch of cars that are all fairly similar in this regard.
I chose mine on the following:
How many miles am i going to do in the car?
Is the engine big enough to make these miles a comfortable experience? - the more miles you do on motorways , the bigger you want the engine to be or driving will soon be a drag.
Make sure its well built - I crossed off anything British, French, American and Italian immediately. Also crossed off all VW's too as the reliability of Golfs arent what they used to be, the spec is Pi** poor compared to other cars in the class, and Polos are just too flippin small (and I have had 2 crap ones in the past). This applies to Audi too - to my mind the dullest cars known to man.
Does it have the necessary toys to keep me entertained for three years? CD Player, Air Con, power steering, Electric Windows, remote central locking and ABS are an absolute must. Climate control preferable and Sat Nav is an absolute gift if you can get it.
How safe is this car? Look at the ENCAP crash test and dont buy anything less than 4 star rating. Some kind of traction control is good here also and as many air bags as they can fit in (I have 9!!).
Who gives a stuff how much petrol it uses as you get the money back in expenses anyway (If not - dont have a company car!)
Should be fairly quick but not too sporty as the ride will kill your back in the long run. 10sec 0-60 or less should be fine.
Has to look good on the drive/out side the house.
Cant stand diesel engines as still noisy when at speed (esp the Audi A3). Usually heavier than petrol and the power delivery is in one area which makes B road driving a painful experience. The extra weight makes the handling even more ungainly than any other front wheel drive car.
Anyway thats what I did .......but then I ended up with an Almera SXE (It has been a fantastic car to drive for last 12 months if a little funny looking), so you can make of that what you will.