For an old 2008 HP/Compaq 6820s, I do not want SSHD but I do want CHEAP. I'd like 7200 RPM in the OLD SCHOOL 9.5 mm thickness. 1: Should I pay five extra bucks to bump from 16 to 32 mb cache? 2: I like Seagate, but would HGST be OK? 3: I've noticed that some ads on Amazon are for "open box", read that "used". Stick that! Can I trust goharddrive to sell only NEW? I'm pretty sure they toss refurbs at us. Thanks for help!