a mono amp can run many subs depending on what resistance the subs load it down, hard to explain, but 2 yes, easily.
yes 500w RMS to each sub which is perfect. To amp the speakers, well it is a better option, but bang for buck many dont see the difference. I'd recommend running them with no amp, see how they go. if your not happy with how they perform add the amp, but be prepared to fork out another $500-700 for something decent installed.