Upgrading Cheap PC

Hey guys, heres the situation. My main rig is going to be out for a few weeks as I am getting new watercooling and I also just need a decent spare pc.
I want to upgrade this PC that I have to a new case and PSU (Maybe Cpu and gpu but theres not much point)
The system is a Dell Optiplex 790 SFF
Specs:
i5-2400
14GB Ram (yes 14 lol 3x4gb 1x2gb)
Dell MOBO (Not sure of size)
GTX 750 Ti, Low profile but I have the full profile bracket so thats np.
80+ Gold 250w PSU From Dell, Proprietary Dell size as the case is small
120GB Sandisk SSD Plus
1TB Seagate HDD

I would love some cheap upgrade suggestions. theres no budget but under AU$120 would be good
Keep in mind its AUSTRALIA so prices are different and so is availability.
FYI I dont mind buying used parts

Thanks