2 old school 12" rockford fosgate he2's in a sealed enclosure that use to be in my dads blazer, when I was younger I liked them, hell they still aint half bad, except he has 4 He2 12s sitting in his room not being used.
I guess you could keep it, but I doubt if its putting out anywhere near rated power. You could just keep your current amp and if you are not satisfied once you get a system going I would then reccomend you upgrade your amp
Have you considered the Rockford fosgate prime 1200 rms amp? It's under 275$ @ sonic electronix and would be perfect for either 1 X 18 or 2 SA 15s and on top of that you it wouldn't draw as much power as the hifonics amp you were considering. If you go that route you should be fine with the big 3 & a 2nd battery