Do i need a sump?

[DEAD_BY_DAWN]

after seeing a few great tank in the past few days,which all had a sumps ,i was considering adding one to my 20g before summer hits,but i do have a few concerns before doing so,
1.do i need one (if its not broke dont fix it)
2.cost can you make /buy one for low cost
3.will it help keep the tank cooler in the hotter months
4.can you just add one to a set up tank with no risk
5.what should i put in it algae,ls,nothing
thanks for ansering any of these questions.
5 1/2.any idea of the cost involved.