Nothing wrong with sponges man. Tons of surface area to play around with.
If it were me i would by the larger sized sponges. Hydro V is rated for 125 gal. Id probably start out with at least 2 amd a decent pump. I like the whisper ap150 and 300.
Theres allot of options. You could even fill a phosphate reactor with sand and create a fluidized bed to go along with the sponges.
Sumps are great but if your not ready... then your not ready, nothing wrong with that either.