As a general, there aren’t any Swedish plus tested group 0+/1 seats that can attach to a pram as these stay in the car. The majority of infant carriers remain rear facing and do well on safety, so I wouldn’t worry about this stage provided you stick to well known brand—think Britax/maxi Cosi/nuna.
Alternatively, there are some car seats that come with ‘modular bases’, which use the same base for both the infant carrier and toddler seat. These can be expensive but useful if switching between cars—eg instead of two toddler seats for each car if you have a base in each then it’s easier to switch the seat between the cars without having to reinstall the seat—although bear in mind this is better for the toddler stage. Most toddler seats don’t need a separate base anyway which can make it confusing when baby outgrows the infant carrier.
If you plan on having DC2 when DC1 is around 2/3 or so, and you have a modular seat that DC1 may still be using, but likely to want the base for DC2, which can work out rather expensive if you decide to buy another base or you’ll need a new seat for DC1, at which point you’ll have to decide whether DC1 is old enough to go forward facing or fork out for a 25kg seat.
The 18kg seats as a general are based of average children on 50th centile to four, or you may find that if DC is on higher centipede or you don’t feel safe putting them into a booster at four (I found that to be the case), at which point you’d need a 25kg seat which can actually be cheaper than modular seats (Cybex Sirona Z is circa £220, but axkid move is a similar price and can last much longer). As far as I know, there are two which have Swedish plus toddler tested seats—the Axkid modukid and BeSafe Izi modular.
Do you know which pram you’re getting? If you get one that’s compatible with maxi Cosi/universal adaptors, then you can use the following infant carriers with it:
All maxi cosi’s
All Nuna
All Cybex
Joie (all except Juva)
All BeSafe
Axkid modukid infant
Recaro (with interim adaptor)
Okay and finally—there is one car seat that can stay in the car but also has an infant carrier detachable which can be used on all maxi Cosi compatible prams with an interim adaptor from recaro—but, it isn’t Swedish plus tested and you don’t have to use it forward facing.