Well, if she is happy to rear face as long as possible Tea's suggestions are good. :) I was only going to say to look at something like BabyAuto Dupla if you think she would want to go forward facing later. But you could look into harness/booster seats which harness up to 25kg later, especially as this seems to be an expanding section of the market at the moment.
Otherwise for up to 18kg you can save money by going for the Joie seatbelt fitted seats - Tilt, Steadi, Stages (would give Every Stage a miss) but the seatbelt does cross the bit where you put the child into the seat, which is a bit annoying.
Spin seats are nice but dependent on the car. Having just recently changed from a tiny car to an SUV, it's made a huge difference - the spin is actually useful now whereas in our old smaller car I was still having to duck in and make sure I didn't bump DS2's head on the ceiling and arguably the spin wasn't that much of an improvement over a purely rear facing seat. It was a bit easier to get the harness tight when he first went in it.
My way of thinking is kind of - rear facing is rear facing. It's incredibly safe regardless of which seat you get, so apart from looking at the rear facing limits and checking it fits your car, it doesn't really matter which seat. Whereas if you're thinking of forward facing at all, that's a much riskier thing to do so you want a seat which you're definitely going to use correctly every time, preferably has as good a safety rating as you can afford, etc.