Alex didn't outgrow his first stage car seat until he was 18 months - he was 9lbs 4.5oz when he was born, and has always been on or around the 75th percentile, so not a little baby. Infant seats are by far the safest out there, so I would try and keep them in there until they've completely maxed it out - either by weight limit (most of them these days are 13kg, but there are a few with lower limit of 10kg) or by height (length of legs sticking out really doesn't matter, the rule is normally that they've outgrown the seat when the top of their head is level with the top of the seat - but again, there are some seats out there with different rules).
We have an Axkid Minikid which isn't isofix, but that means that it can rear face until 25kg. It's fixed using the seatbelt (really not tricky, even my parents can do it), and tethers which go to the runners (or other suitable tether point) of the seat in front. Attaching them is probably the trickiest bit, but you can buy spare sets of the tethers so that you don't have to move them each time you move the seat. You can also get seats that have both isofix and seatbelt installation so that you can use isofix until they reach 18kg and then only have to use the seatbelt after that. Also, not all seats necessarily fit in all cars (this is true of forward and rear facing seats) so you need to check which seats fit in your car or cars if you're going to be moving it about).
There is much more of a range (and the seats are cheaper) if you only want to rear face to 18kg - we didn't because our son is quite large, and I didn't want to have to turn him before he turns four if he had reached the weight limit.
When he gets to 25kg and outgrows his current car seat (which I really do hope won't be for at least another 3/4 years!) he'll be going into a high back booster (or normal booster if he outgrows a high backed booster) until he's at least ten years old because apparently a child's pelvis isn't wide enough to use an adult seat belt safely without a booster until then. Haven't really researched this bit much yet though, as he's only just turned two, so still have a while!
Did soooo much reading into all this when A was going up a seat (after hearing about ERF seats from somebody else on here - had never heard of them before), that I always feel like I ought to share all the stuff I've read in case it helps somebody else!
Do feel a bit like this though:
so I'll get off my soapbox now
I'm sure by the time he's 2 he will be wanting a new bag of some cartoon or skmethjng or other 
