In reply to Mancclimb:
I did a thorough review of the Deuter Kid Comfort Pro last year, which I'd really recommend (link below). We had a much softer one prior to this and infinitely prefer this, as the extra support, particularly on longer walks, makes a massive difference to your comfort (and likely the child's too). That said, there's no denying that this does come with extra bulk, size and weight as a result, but thethe trade off has been worth it.
When it comes to buying second hand, if you find the right thing then this is definitely worth it. If not, it just depends on how much you want to compromise. If buying cheap means buying twice, or getting something that doesn't quite fit, or is a bit knackered, then maybe it's not worth it - particularly if you're going to be using it a lot (and we've certainly used ours loads throughout the past 12 months).
I can certainly see this carrier having more than enough life in it for as/when/if we ever decide to head back into the fray and have a second child.
Whatever you decide, good luck in getting out with the little'un!!
https://www.ukclimbing.com/gear/rucksacks/deuter_kid_comfort_pro-12751