Ok so i have DD1 (5yo, 48 inches, 50lbs) in a britax adventure high back booster (belted version), and DD2 (20months, 32inches, 22lbs) in a BeSafe izi combi X3 (isofix version).
We're planning ttc #3 when DD2 turns 2 this summer.
The BeSafe is good from birth in terms of weight, but recommended from 6months due to the seat angle (which flops heads of many babies forward). We had a very rigid 12 week old when we bought it and she went straight into it with no problems with her head position (infant insert in and seat in max recline position). For the first 12 weeks we used a bucket which came with DD1's old Graco travel system (mosaic).
That Graco seat is now definitely too old for us to use it again, so we need a new seat. The options are:
Buy a good quality 0+/RF to 28lbs seat for the new baby and then move it to the BeSafe and DD2 to a FF booster (it's hard to guess when #3 will be 13kg but DD2 still isn't at 20months! DD1 grew heavier faster but they have different fathers and DD2 and #3 will have the same father. Anyway, assuming #3 hits 13kg at 12months DD2 will be at least 3yrs9months old).
Buy a cheap infant bucket to get us through the first few weeks then get a second BeSafe for #3.
I'm having trouble deciding because DD2 is such a peanut just now, she may well fit within the BeSafe limits until she's 5 or more, and i'm not sure i'd FF before then ideally, but then the BeSafe + new bucket will be SO expensive...
I've noticed this forum is heavily N. American-centric, so I don't think it's that your question is being deliberately ignored...it's just that probably no one knows a good answer for you! LOL. I'd suggest checking out the international forum at www.car-seat.org (I think it's called Canadian and International Issues). There will be a broader range of technicians there who will be able to help you out :-)
I'm assuming you're in the UK, since we've got the britax adventure too.
I would look at the group 0 + 1 seats. I think they give a bit more room than the infant seats, so can be used a bit longer even with the same weight limits as the infant seats. It might give you a bit longer for your DD2 to be ready to move to another seat.
We had the britax first class and were very happy with it.