It probably depends on whether you plan to buy locally or across the water. Across the water, given the budget mentioned, then its likely that the newest car you'll buy is around the 2012 mark and either will be crucified on the NOx levy (although its worth checking, I think a BluePerformace optioned car may have EU6 level of NOx so that would take the sting out of it) and I'd expect other taxation changes over the next couple of years to further disincentivise a diesel of that era - e.g. road tax adjustments to reflect NOx and duty changes to remove the price advantage at the black pump.

Another thought, how about an F10 520i? Similar bhp to a 520d but less torque and slightly higher CO2 would put it in the €390/23% road tax band, higher than a 520d. The NOx figure for the petrol is negligible though. Just watch the spec to ensure its not pushed over in to the next VRT/road tax category (€570/27%) based on CO2 - auto beats manual and smaller wheels are better. A manual with 18" wheels or bigger will push it in to the next category, but an auto with any size wheels should keep it in the €390/23% category. A 528i auto (2.0) on 17" wheels should also just scrape in to the €390/23% category but anything manual and/or with larger wheels will flow over in to the €570/27% category. Both are relatively rare compared to 520d so you might have to scratch around to find the right one.

Caveat - always check the V5 for CO2 and NOx before committing.

Any 520d that's in your budget will have the potential for timing chain issues, if they haven't already been changed by now. I stand to be corrected but I think the e60 535d still had the M57 engine so the main issues there are likely to be EGR, swirk flap, etc. type issues but I think the chains were generally OK on them - again, I stand to be corrected on that.