Hi C30. I assume you have decided on the C30 in particular (rather than the S40, V50 or C70) ?
Reason for asking: from what I've seen the C30 usually had cheaper options (eg cloth trim and manual trans) to keep the cost low whereas most of the S40s had leather and auto...but are now cheaper than the C30s.
We have had a few of the AW55/51SN transmissions: I learned about ours when the 2003 S40 had a "limp home" mode at around 50,000km. Fluid flush a few times and reset adaption, and the transmission was perfect for the next 100,000km (with flushes every 50,000km). I couldn't believe the state of the fluid after the first 50,000km - we bought the car near new at 30,000km from a Surgeon, and the only person on the planet who drove slower than him would have to be my wife. Volvo say the transmission is "sealed for life" - there will be a few experiences on here of leaving the transmission alone for 250,000km but that's not my experience. As Anorak says they're not the most refined transmission but do the job quite well - I understand that the fundamentals are quite robust (the transmission is an Aisin Warner (owned by Toyota) and is used in many other cars https://en.wikipedia.org/wiki/Aisin_AF33_transmission but Volvo software punishes the hardware more than other manufacturers. Any wear particles in the transmission wear out the valve body and stick in the solenoids causing havoc. I don't believe the transmission is any worse than many modern electronically controlled units, but I'm possibly wrong there. I've never had serious issues with them.
We have a S40 T5 AWD with the same transmission and it gets a much harder life. I flushed its transmission at 100,000km and have done it again every 50,000km and no problems so far. You need JWS3309 fluid. It's not too hard to find, but be careful most garages will just refill with whatever they have in the bulk tank. The AW55 transmissions don't like it very much.
Ignition switches can cause problems - ours is failing now and they're stupid difficult and expensive. Yes coolant bottles need changing now if not already done - there's a level warning on the washer fluid but not on the coolant...grrrr... (yes I'm on the second engine because of this...do not overheat these units !)
The A/C compressor clutch is likely to be hit on a kerb, and the clutch will be badly worn or need reshimming, the timing belt needs to be up to date and replaced with pulleys , idler and tensioner (I've never seen a belt go yet but the tensioner fails often - it is the part which really needs doing)
Suspensions only last around 150,000km - but they're made to be replaced often and easily. The C30 suspension is shared with the Mazda 3 and Ford Focus of the same year - so suspension parts are actually easy and cheap.
Radios commonly lose AM reception. It's in the display - and expensive to repair. These are not cheap cars - they are extremely complicated and the number of unique modules which have to play well with each other is astonishing. You can't treat it like a Corolla or Camry, and you really cannot ignore little details.
Don't be surprised if your cars are missing the cover at the back of the handbrake, or the grommets around the door lock buttons. I have no words...cheap bastard interior designers...
Lastly, the PCV system will at some time need work - if you're lucky it's a $10 disphragm but unless you're mechanically minded your local gorilla will saws-all your air cleaner mounts off because he couldn't be arsed googling for 5 minutes. (Your ownership experience will be much better if you find somebody who understands these cars and don't try to "nickle-and-dime" them) They're actually fun to work on if you're that way inclined...
The 2.4 should be quite bullet-proof. You need to remove the inlet manifold to replace the spark plugs which I find to be an entertaining piece of engineering, but it won't stress the transmission at least ! A T5 would be more entertaining, and seem to be cheaper as well !
I don't have any experience with the Diesels - I just don't understand their attraction. They aren't cheaper to run, are coarser and noisier. If you want a tractor, just buy a tractor.
Have fun - and be picky. There are plenty of these things around. Many of them lived a pampered life in parking garages in Mosman. A lot of others were showing up Ford Focus' at Mt Druitt. Find one of the ones which has been looked after, and it should look after you.
Good luck (if I haven't put you off already...)