no sponge filters wouldn't cut it. for filtration I would do the fluvalf fx4 or fx6. I have bought a used tank, I filled it up for 24 hours, also check silicone seal and make sure its still intact and reliable.
If i were to reccemonds any stocking ideas, I would do defentely get a...
arowana
datnoid
sevrums
oscars (As you mentioned)
blood parrots (as you mentioned)
Peacock Bass
Here is a video I watched when I picked up my second hand tank, its very useful...