A new 750.1 is not worth it if your cousin is giving you a solid deal on the 500/1. Also yeah the 750.1 is 750 rms at 2ohms. You want to run it at or around the subs rms, which is 400 rms. The 500/1 should be a good match. 750 rms, is almost at the subs peak, you will destroy the sub. This is if the subs a dual 4ohm, it will be running off 2ohms at the amp (which is 750 rms).
depends on the ohms.. but overall with that one sub, I would use the JL Audio amp. That kicker amp is too much for that one sub, unless you have 2 cvrs.