Here's an easy tutorial for a quick Doctor who costume.

you need:
messy hair
a brown trenchcoat
a grey or blue pinstripe suit (i'd recommend a Cedarwood State one)
a red tie
a sonic screwdriver

Step 1: The Hair and Glasses

(the glasses ARE optional)
David's hairstyle is like a upward left sweep.
start by coming your hair upwards and then gelling it once you're you've got the windswept look.

Step 2: The Trenchcoat

it's a typical brown trenchcoat, you'll probably be able to pick one up at a charity shop for a pound.
i used the same trenchcoat with my Duke Nukem costume at the zombie walk.

Step 3: The Suit

unless you're a tailor, don't complain saying this isn't real cosplay because i don't have a masters degree in tailoring.

There are 2 suits you can use. a grey pinstripe suit or a blue pinstripen suit. i've gone with the series 2 look of the grey pinstripe suit and bought a suit from Cedarwood State for £50.00. you'll also need a red tie, they're around £2.50 and the blue shirt was about a fiver.
it's £57.70 for all of it.