18 months isn't long for that kind of damage! We've had our two years and it's as new.

I just wanted to say if you get the basinette, you won't be able to use the bottom seat as the basinette sticks out too far. Also, I think the reason for only going up to 15kgs in the bottom seat is to stop it tipping - a newborn won't have enough weight to hold the front down.

I wouldn't have any issues putting baby in the bottom though. I have a friend with twins who has this pram, and the babies swapped which seat they were in with no issues. As long as your DS faces forward so his feat are out of the way you should be fine.

Another option is if you're into baby wearing? DS goes in the pram and baby in the sling?