I have a radian rxt, we picked it because of this forum and market research, but it also has one of the higher ratings for rear facing as we wanted our little one to stay rear facing as long as possible. The metal frame is one of the thinner ones, and one of the features is that you can fit three across (not in the ISF as there's no middle). The rxt does not have a fixed base, and to be fair I'm not sure many others do. You'll be fastening them in with either the latch system up to a certain lbs and then seatbelts after.

We've taken it throughout the US and also parts of Asian with no real complaints other than airlines damaging them. If you're going to be rear facing keep in mind almost no vehicle is going to have tons of space in the front seat. In the ISF the front seat would be regulated almost completely forward, but then that's true in our other SUV as well. Radian has an angle adjuster that gives you some more room, but the front seat still isn't what I'd consider spacious. The rxt is rated to 45 lbs rear facing, again one of the higher ones when we did the research.