We started with a Britax rear facing seat, very secure fit -without the base- only using the lap belts in my SC. This was good until around year year old.
He really enjoyed getting into the forward facing seat. I decided to buy this ROMER prince from Europe, this was a good fit also, some people suggested this design was unsafe, but I think it is probably fine.
When he outgrew this seat I bought a COSCO (you can search the forums for the model) which was a good fit but you need to have the upper tether mount installed for a secure fit.
There are plenty of very informational threads on Pelican and Rennlist on baby seat installation for various 911 models.
Good luck, the 911 can be a very practical family car (at least it was for us)!