It really does depend on many things - insurance could be anything from £30 pm to £500 pm, although unless you are a teenage boy with a gazillion points working as a pimp, then it will probably be at the lower end of the scale!
DP has a fabia, 2001 model. Its not the best I have to say, in terms of fuel economy, and tax, but it depends what model you get. I know the 1.9l diesels are recommended. He has a 1.4l petrol. He does 280 miles to £60, but he does a lot of short journeys. It is very roomy as a small family car goes however.
I have a 1.1l clio 2006, it cost me £3000 nearly 2 years ago. Tax wise its been ok - £71 every 6 months, but petrol wise, its great, £60 to fill up, and can do up to 500 miles. I've done 30,000 miles in mine, and its done 58k since new, and hasn't needed anything yet other than 2 new tyres (one burst). Its been worth it for me, and I won't replace it until it becomes uneconomical. Oh, and I can fit a weeks food shop in the boot easily.
I would say you probably need £150-200 a month to cover insurance, tax, petrol and repairs/mot. Depends on how many miles you are doing also.