That would buy you a nice Duratec, Zetec, C20XE engined car with a bit of tuning say 190 to 240 bhp. Take little notice of the year and mileage as many Westfield's have been up dated, rebuilt and you'll be checking the condition of the bodywork chassis and trim when viewing a car.
Popular upgrades that add value are widetrack steering rack, carbon fibre body parts and dash, decent wheels and tyres, modern seats and harnesses to name a few.
You'll need to decide if you want a road or track focused car as the options and upgrades are quite different.
If you're a keen DIY type, it maybe beneficial to buy a basic no frills Westfield for £10k and upgrade it yourself to make it your own.
Have a trawl through past classified ads and see what's going for your budget and prices do go up in springtime so be prepared for that in a couple of months.
Happy hunting.