DW has closer to 800 episodes (though some are missing believed wiped). But if you start from the 2005- era you're right it's about 100 episodes.
But the point is to get Torchwood background - at which case you might as well stop at the end of the 10th Doctor's era.
As was every series opener bar Series 6. There's so little it's not worth watching a million episodes just for that. Wikipedia's for that. Just watch for a good show.
Reset/Dead Man Walking/A Day in the Death These three episodes kind of bleed together. I thought the case of the week in Reset was a bit dull, but the story really picked up when Owen was shot. I don't know what they hell Torchwood encounter in DMW, but I have my doubts it was really Death... more like a Death monster. Looks Owen will be sticking around as some kind of weird undead human. These episodes did a great job developing Owen though, we barely learn anything about him in the first season except for the fact that he is a womanizer. It was interesting how his character dealt with being dead and being a different kind of immortal compared to Jack. I really liked his conversation with the dying old man that collected alien artifacts. There were Owen/Tosh romantic vibes galore, but I think they should have introduce this plot much sooner. Also some woman called Martha Jones is in these episodes. I guess she was one of the Doctor's companions? Something Borrowed I loved this! We badly needed a comedy episode after the last three depression filled installments. Gwen and Rhys finally get married! Yes! I'm really starting to really like the Rhys character, and in his own way he has started becoming the Chief O'Brien of the series. He started out a little too much of a buffoon, but he was finally given a chance to grow as a character when he learned about Torchwood. I thought the actor always did a good job with the thankless role and I guess the producers thought so too since they kept asking him back. There is some crazy stuff with an alien pregnancy, but all I can say is that Gwen looked amazing in her wedding dress even when she was carrying the alien hell spawn. It was fun meeting Gwen and Rhys' parents. I was REALLY hoping Rhys would save the day by killing the alien with the chainsaw though, that looked wicked. Gwen and Rhys really stole the show in this episode. From Out of the Rain A dull monster of the week episode. The two ghosts were a little cheesy. But the flashbacks were nice I guess. Adrift A very well written but depressing episode. The episode started out as another missing persons episode and just snowballs into the sad reveal of what really happened to the missing teen. Rhys got another chance to shine with his speech to Gwen about why she is in Torchwood in the first place, and also when he was supporting Gwen at the end. Best part about the episode was that there was no cop out ending. Torchwood can't always save the day, but they can try to make things right.
Yup, in the third season of the revival series (the one that ran contemporaneously with Torchwood's first season), plus occasional return appearances thereafter.
It seems the biggest problem behind series 1 of Torchwood may have been that it had an incredibly truncated production, it was rushed in to production and went from concept to screen in something like 9 months. So they really were still working out what the show was while it was in production.
Daer old Auntie Beeb was obviously happy with it as it was promoted first from BBC3 to BBC2 and then to BBC1.
And in addition, the original producer wasn't happy with how things were going (whether that was 'creative issues', or just that as a director stepping up to his first producing job he found producing didn't suit him isn't clear), so he dropped out and went back to directing, with a replacement coming in while production was underway. Season two largely carried across the same creative team as season one, so they'd got into their stride by then. Then it's all change for season three, as creator Russell T Davies takes a more hands-on role (having been too busy to do much more than write the first episode until then).