Many people in your situation buy something like a Citroen Berlingo. A used Berlingo represents good value for money while it should also be cheap to run. There are a number of ways you can convert it into a comfortable microcamper - from a basic 'boot jump' to a more luxurious conversion. You might be able to save time and effort by finding a pre-converted example. And as they're always popular, you're unlikely to lose a lot of money if you decide it isn't for you.

A Volkswagen Caddy could be a good alternative - although the VW badge does attract a price premium in the camper market. We really rated the Caddy California when we tried it: vans.honestjohn.co.uk/van-reviews/volkswagen/caddy.../

Alternatively, you could look at a more conventional car like the Dacia Jogger. The Jogger is a very practical (and affordable) seven-seat car. You can even order it with a Sleep Pack which adds a storage box that doubles up as a base for a mattress. It's a really clever idea that could be ideal for your needs.