Hi Andrew,
Most modern cars have loads of electrical sensors and if something quite minor needs attention the EML comes on. Getting a diagnostic tool such as an OBD2 side scanner [ £20 off eBay] will tell you the code, a quick google will tell you what it means, and another quick request to the club members will tell you the solution.
My son has just bought an X Type Sport estate 2.0D with 91k on the clock on an 06 plate for £3,250, so I am sure you will find one within your budget.
Regards, Peter.