Depends on what you want to keep. I personally would step up to a 10g because you can find filters and plants for it easer, many more options. That light would be ok on a 2.5g. As far as filtration, you'll need; carbon, ceramic( bio ), and foam ( mechanical ). Be sure to clean the foam weekly as it doesn't take much to raise ammonia in a small system like that. You can take an ol plastic bottle ( show in image ), cut out the bottom, fill it with media bags, and run air up it. Or theres this:
Azoo Palm Filter . But a 10g. That would be better. I had a planted 10g once;
-10g tank $10
-substrate $6
-Thermometer $2
-Heater $10
-AC filter $10
-CO2 $DIY
-Light $30 (( DIY upgrade ) 28w )
Total:$68
10g would be more to look at too.
2.5g:
-Tank $8
-Substrate $4
-Thermometer $2
-Heater $8
-Filter $8
-CO2 $DIY
-light $60
Total $90
The CO2 on a 10g was done like this; I took a 20oz pop bottle, drilled a 1/4 in hole in the cap. Ran air line 1in into it and siliconed it there. I hooked the other end onto the filter intake. When I made the CO2 concoction for the big tank I added some to the bottle, filling it half way.