There's never going to be a guaranteed reliable motor when spending £4k or even £10k, things go wrong on cars no matter what they are or how they've been treated!
Best bet is to go for the best of the bunch with the least chance of failures, but more importantly the costs of repairing them if they do go wrong.....
Millions of cars from Seat, Skoda, VW, Audi all had that 1.9TDi lump in so loads of people know how to work on them, how to get them going best and most importantly parts are very very common so not too expensive to get....
I'd personally go for an Octavia 1.9TDi or a Passatt 1.9 TDi if it was a car for racing.
Get an 06 or older without a DPF and the old PD TDi engine and you'll have no problems, they're loud and sound like tractors but they give amazing economy and still go really well...
You can't go wrong with that in my book - The BMW 330D would be lovely, but to service, maintain or fix anything it's going to cost you a hell of a lot more then most other brands.....
Choose anything with the old faithful VAG 1.9TDi (Pref 130+) engine and you will be happy I reckon
