What they said. If you are not a huge spender and if you can't get sign-up bonuses, it will take you years, but...

Do start working with 700, but not with City. Try good Chase and Amex cards and go in slowly. Do not do any app-o-Ramas for now and try for 1-2 cards at a time. The funny thing is--contrary to popular believe, your score might very well improve with every new card despite the pulls.

You can pay mortgage by plastic but it'll cost you something in the neighborhood of 3%. The service I know is called Charge Smart. I would never do it for rewards, though, because the fees will negate any benefits.

And also, it's my personal opinion, and I know most people here will disagree, but don't get obsessed with premium class flying. Seriously, at least until you have a few hundred thousand miles under your belt.