I started writing this before others posted their replies, so sorry if any of this is repeated but I didn't want to put any of this to waste.
Having been to Thorpe Park 21 times (the 21st time was today) I'll be able to answer these.
1. A single fastrack for the larger rides is £6 and for the smaller rides is £3.
2. Colossus, Stealth and Nemesis Inferno have front row queues. Swarm doesn't have a front row queue, but as there aren't any staff at the gates you are allowed to wait for the front. Saw doesn't have a front row queue either, whether you get to choose your row really depends on the staff (some days you just walk to a row of your choice, other days you're assigned a row).
3. You only get to choose which side you go on when it's not crowded, sadly on busy days they'll assign you to a side.
4. If you want to start softly, ride Vortex as it is very graceful and will get you into the motion of flat rides. If you want to start big, ride Rush or Slammer as they're the more intense flat rides (which will get you into the motion).
5. Lost city queues tend to build up quite quickly and become rather long so I would recommend that area.
6. The indoor drop, I'm not gonna lie, it's an intense drop. However, it's over very quickly so you will not be struggling to breath or blacking out halfway down. It does mentally prepare you fully for the outdoor drop (the 100 degree drop).