Australia is pretty terrible for price too. Nissan GT-Rs are about five times cheaper in the UK with the current exchange rate. A friend of mine is selling his mid-low km mid 2000s Audi RS6 in the UK for about the same as I bought my 10 year old (at the time) Honda Integra in Aus. In Australia an RS6 of that quality is worth 3-4 times what he's asking.
Anyway, here's my advice - keep in mind that this is based on someone who's mainly interested in smaller 80s-90s JDM cars, and I live in AU and the car markets might be vastly different.
Avoid 2000s Mercedes unless you really love the drive. They've only recently started over engineering again - something they never should have stopped doing really. I dunno what it's like in Singapore, but in Australia European cars are pretty much a waste of money - you get a LOT more for your money out of Japan - though it does depend on your tastes. The Accord Euro is an excellent car with a nice 2.4ltr engine in australia (quick too - though with clutches that like to burn up when driven hard). The Accord may come in a 2.0, 2.2 or 2.4 over there. I'd avoid anything from France unless you don't mind replacing interior trim semi-regularly. I must admit a soft spot for Alfa Romeo 159s - incredibly classy though the historical reliability of Alfa's is not inspiring. Lexus makes a range of luxurious, reliable, but incredibly boring cars.
Best way to choose a car is to drive all of the ones on your short list. I'd suggest at least two different cars of each type - two second hand cars of the same model can be extremely different based on how hard their life has been. Make the choice of which car model to get afterwards, and do it at home NOT at the dealers. Then you just have to try various different cars and find one that has the features / has be treated the way you want.
The BMW 520i is carrying more weight, so you'll probably feel that when driving it compared to the 320i. I did, however, just do a power comparison and it comes out very similar to the 320i in power + torque to weight so it might not be bad in that regard:
BMW 520i (manual): 125kw, 210nm @ 1535kg = 81kw & 137nm / tonne
BMW 320i: 110kw, 200nm @ 1445kg = 76kw & 138nm / tonne
However, I find my car has very low torque, and requires lots of revs to move about with any urgency at all, this is how my car comes out in comparison:
DC2 VTiR (Manual): 125kw, 170nm @ 1150kg = 108kw & 148nm
But obviously if you found it was okay, then perhaps we just have different ideas about what an acceptable level of power is. If you decided to go 520i, get the 2007 model as that has significantly better economy.
If you're thinking of importing - IMPORT FROM JAPAN! They practically throw away brilliant cars every day. Don't bother importing from Australia - our car market sucks. NZ has a good car market, but that's because they have very light import restrictions and they get all their stuff from Japan, so go to the source.
By the way, not all manufacturers stick massive engines in their sports capable / luxury cars. You do, generally, have to go smaller than a 5 series though.
Oh, and whatever you get, get it in Manual.