Yes, that is a better option but be careful there are plenty of kenwood knock-offs in the market, pioneer knock-offs I haven't seen though, also check carefully before buying the AMP whether it is repaired or not. You can tell just by looking at the smudged screws of the amp if has been opened or not. What you should do is look for a used korean amp.
There are plenty of good 12" subs available in the market, you can buy the Pioneer TS-W5000SPL 12" sub for 24k :P, but what it comes down to is your budget.
15" or 12" well that depends on how loud you want your system to be. A 15" inch sub will tend to be more louder with the same amount of power as it will have more surface area and a higher sensitivity rating.