Hi, not sure where mATX comes in as the Sonata is an ATX case, which is good as means have huge choice of PSUs.
If you you can get it and budget stretches to it I would look to Be Quiet! 350 gold.
This should be efficient, high quality and quiet PSU that's fit n forget.
As a cheaper option I used a ColorsIT Black Edition PSU in an i5-650 + GTS450 system I built a while back and not heard back of any issue. Not the quietest by any means but a definite cut above the cheapest junk PSUs while being fairly low cost. Should respond well to a fan swap, can't remember which model I used, 450w I think, or if it has any fan control. Example of 480w model.
You don't mention what the system spec is but I'm sure quality 250/300w PSU would be enough. I have run my rig off a Seasonic S12II 330w, stressed and pulling ~320w from the wall, ~280DC from the PSU and was fine!
80mm fan PSU not necessarily bad, depends and the implementation, they will contribute less to case cooling but that may not matter. My PSU has (swapped) 80mm fan. Idles <600rpm and loads 800~1000rpm...