1>The dealer is either trying to trick you or He's plain just goofy. You don't need to drive a sub at max power always. Do you always drive your car at max geared speed? No.
Driving a gadget (In this case a subwoofer) at lower than max power leaves lot of headroom. Which means longer life & Cleaner sound (No stress).
Let your Baby arrive. We shall check the user manual and fix a decent sub.
Meanwhile- DIY Dual woofer setup (Hidden and Underseat)
2> About Other accessories, it depends on what you need and whats useful.
I'd suggest dropping a Quality 4300K HID kit in the Projectors. That's almost compulsory.