If the OP budget is up to €8k for an '08 executive diesel... it should possible to find one within budget (be that a cheap one that you get the work done on, or one with work done and a full warranty).
Personally not a fan of the 320 and 520 d either (engine issues not only thing that plagues them).
A4 / A6 with a 2.0tdi is hit and miss... up to end 09 you can run into issues with oil pump drive key wear (although there were cars during this time fitted with the longer proper drive key for the pump, but no record of what car has what fitted from the factory). If caught in time will still be a 1800 repair at an independent or double that at the stealer as will need oil pump balance shaft unit plus turbo fitted). If not caught in time it is scrap as would need hone, rings, bearings as well. That said, if you have one without symptoms then it will "only" be around 900 lids at an indy to get the oil pump balance shaft unit with the new key fitted as a preventative measure. Nine hundred sounds bad, but worth it for the peace of mind and just something to be taken into account when buying / negotiating!
Of course Mercs have their own issues with their diesel engine of that era with injectors and carbon build up etc, not to speak of the autobox gremlins and general electrical gremlins.
Moral of the story is: if you want a German exec car of around that age, especially with a 2.0 diesel sized engine, you need to buy with a very comprehensive warranty... or so cheap that you have a few grand in the bank to sort it out when it goes wrong.
The bigger engines of all three of those makes are a lot less trouble (6 cylinders diesel from VAG, MB, BMW all pretty strong and way better to drive than the smaller ones... and typically not that much more thirsty). If it was me I would look at an 09 bigger diesel of those makes, as they will just about slip into budget. Check the tax on them though online before buying as there's a huge difference between cars of the same year depending on the map and ignore any VAG 2.7tdi as they were never mapped for the new tax system.
For example (but there will be more if you are patient and look around):
https://www.donedeal.ie/cars-for-sale/bmw-5-series-3-0-litre-diesel/14862032 2008 3.0 diesel tax 750 58k miles and should be on budget after negotiation