Played this exact game before I bought my current car. Answer in my experience was a Bora or Toledo; marginally more expensive but a better drive and marginally more economical is a 2002-2003 Ibiza 'Sport'. Octavias do exist in PD130 form but 'rocking horse st' comes to mind.
From my experience the Leon tdi se was the cheapest when SWMBO decided she had to have a VAG diesel (god knows why but I didn't argue, not a bad car as it turned out).
Bora is a nice enough motor. We've still got one on the fleet (PD130, Sport trim), 9 years and 93k of pool-car use and it's still tight as a drum, and still returns close to 50mpg.