I have heard too many horror stories about the 1.9DCi to ever be tempted to buy or recommend one.
I did have a laugh last night while blasting up the motorway upon passing two very slow cars in the slow lane. One was a MeganeII with its (very dim) hazard lights flashing (no other lights, it was dark!) being rope towed by a LagII 1.9DCi. I did wonder if they would get to where they were heading.
If your brother is covering 300+ miles a week he needs something very dependable and so I would not chance it with a 1.9DCi. Ok so if it goes wrong he might not lose much money, but the potential inconvenience of breaking down, having to break the car, being without a car and needing to find a replacement are far too many reasons not to get one. It is a shame as otherwise I really like the LagunaII, petrols are the only way to go with one of those.
There are a good amount of diesel cars available for £1000 and it is hard to ignore the truely excellent 2.0HDi 8v engine. Id recommend a 406, Xantia or C5 with that engine, even a Xsara. Its a well proven engine and just goes on and on for many hundreds of thousands of miles.
The Mondeo is not a bad car, I found a lot of rusty ones (2002-2004 examples) when I was looking a few years ago. The 1.8TDCi does like to eat injectors too, but its a half decent car, there are so many of them about and it will do as a commuter.
As for a Volvo, im not sure, by all means reliable but the 1.9D S40 is supposed to be pretty unrefined and the larger cars with the D5 will be superb but quite a bit heavier on fuel.
I would go with a 406 or Xantia for that money, simple, a good amount to choose from and the 2.0HDi engine is one of the best.