i would say if you don't want it too strong, go with 1 10" or 1 12" depending on if you want bass that penetrates you or bass that just vibrates you. you need an amp and an amp kit (power wire, ground wire, remote wire, rca's). if your HU does not have rca outputs then you will also need an LOC (line out converter) and splice that into your rear speakers. like the system i have, i have 2 12" SQ subs at 400 rm each, an 800 rms 2ch amp, and it sounds very nice. i spent maybe 500-600. it just depend if you want SQ or SPL.
send me a pm if you have any question.