Does the Simplican work across both 1st Gen and 2nd Gen?

Short answer, yes.  SS3 updates OTA and is encrypted.  SS2 is hard coded, no updates available so base, sensors, keybad, siren, are not compatible.. Cameras, doorlock will work with both.  Someone else will have to comment on the Simplicam weather resistant kit.

Cameras work on both systems.  They are effectively independent of either, really.
Door locks do not.  They only work with SS3.

You can find the outdoor "wetsuit" for the camera here if you scroll down: https://simplisafe.com/simplicam-security-camera
Note that some people have found that the edge around the lens needs to be trimmed flush to prevent "flaring" from the infrared illumination.
Here's a pic:  https://simplisafe.com/files/images/camera-landing/outdoorkit_dt_b.jpg
Here's another: https://simplisafe.com/files/images/camera-landing/outdoorkit_dt_w.jpg

Note also the camera still shouldn't be directly exposed to weather, IMHO.  Put a little roof over it, if you can't put it under one.  That said, it appears to handle moderate temperatures just fine.  I've had one on my front porch (under the roof) for approaching a year, from single digits to 95 degrees F with no problems.

I just noticed a fib on that page:  "SimpliCam's intelligent detection algorithms are calibrated to detect the unique heat signature of humans, so the alerts you get are the ones that matter."   Baloney.  

I have a recent nighttime video of a fox that came onto my front porch.  Last I checked little foxes and big humans are not very similar.  I also have one of a wasp that flew up to the camera!  I like Simplisafe's system, but their marketing department lies like a rug...  And truthfully I like getting videos of anything warm that come on my porch, human or not.

The sensors are "change in heat".  If heat changes, the sensor is triggered.  something small and warm close, or something large and hot far away, doesn't make any difference.