Watching it from S5 is definitely something I'd recommend, but don't feel like you
have to. Not yet, anyway. What I think would probably be worth a go, though, is bringing up The Impossible Astronaut on iPlayer.
While I'd say that The Eleventh Hour (S5E1) is basically the perfect starting point, Doctor Who is designed so you can start from more or less
anywhere. Each episode works as its own story, and so there's no harm whatsoever in beginning with The Rebel Flesh. You shouldn't feel like you're missing anything if you come into the story a little late. The series starts anew each week. There's an ongoing storyline that's occasionally hinted at, but you'll be able to work out what's going on no matter where you begin. No matter whether you start with Vampires of Venice, or Blink, or even the Waters of Mars, you're still on the same page as everyone else. That's absolutely unique to Doctor Who as a serial drama, and so you should never feel like you're obliged to read up on the entire saga before you start watching. You don't have to wade through oodles of backstory before you're allowed to watch each new episode as it unfolds, you're perfectly at liberty to simply just sit back and enjoy the ride from The Rebel Flesh carrying onwards into the future.
However! The underlying arc of
this series is absolutely brilliant. It's very exciting, very clever, introduced in one of the best-written stories the show's done, and it's one of those brilliant mysteries that you don't
need to follow, but I think you'll really enjoy tucking into.
Right here. The show makes sense without it, but it's so much meatier with it.