TBH at 2000 miles the difference between the most frugal diesel and the thirstiest petrols will be £100 or so. It really won’t be worth paying the premium for a diesel. That’s 3 tanks of fuel, 4 if it’s a small petrol tank.
Reliability, again I don’t think that’ll be an issue in any modern car. If something goes wrong in 10 years doing that few miles you’re unlucky.
Given your criteria I’d look at cars that have already done 150,000-200,000 miles. And only spend the minimum you need to (say upto £1500 for something from a posh brand, or £600 for a Ford Focus). No point buying a low milage car and losing value on it doing nothing. May as well just buy something that might last 10 years, or might not, and keep the other £6k just in case.
If you still want to spend all that money, get a Berlingo.
If you don’t want to spend all that money, get a Berlingo for £600. The 2.0 HDi has all the refinements of a tractor but fits your criteria of being repairable. The 1.6 HDi is more modern, but more temperamental. But Id still suggest you’d be unlucky to ever break it at that mileage. Petrol versions are less popular, even cheaper, and pretty much bulletproof.