Take what I say with a grain of salt, considering I only have one kid and she's six months old. But, I'm just thinking of the logistics.
First, I'm assuming the seat that your DD is in now is out of the picture and is not going to be passed down? Or, it's a bucket that will only be passed down for a short amount of time. So, I'm not going to enter it into the equation.
Option 1: So, if you get your DD a seat that RF to at least 40 and FF to 65, that might be the only harnessed seat she'll ever need, depending on shell height, right (not sure on that... since my baby is so young I haven't paid that close attention to older kids' needs)?
And, then you could get new baby the same thing- a seat that would be the only seat ever.
So, that's possibly only 2 convertible seats you'll ever need.
Option 2: You get the seat you like best, but maybe it only RFs to 30 and FF to 45 or something.
So, your next baby will also need a seat, and then maybe you get another seat like your DD's lower limit seat.
Eventually you have to buy your DD another harnessed seat. But, perhaps by the time 2nd kid needs a new seat your DD would be out of her higher limit seat and then could pass it down to 2nd baby.
So, with option 2 you're looking at at LEAST 3 seats total, but maybe more.
But, with option 1 there are a lot of unknowns also, such as will a seat come out later that has an even higher limit that you love. Also, by the time your DD is outgrowing whatever seat you get, by then there could be another seat that has an even higher limit.
Also, do you have a car that will fit the really high limit seats? I have a myride in my little kia, but I think DD will probably outgrow that by height before she outgrows by weight.
Hmmm. What I've realized is that I'm not any help at all on this matter and shouldn't post this response!