I bought this stroller second hand (in Denmark) but now I worry if is contains chemistry that might harm my children since it is not the newest model.
Where do I find information about the substances this stroller contains?
The bad news is that you won't find much about that, unfortunately. Strollers go through a lot of testing when made, so, on the other hand, I wouldn't worry much. Very few strollers actually contain bad chemical compounds and those few who do usually were either faulty or very very cheap, somehow going around the needed tests.
Also good to know is that this is a global, high-quality brand, so the tiny risk is even smaller with Mountain Buggy.
And lastly, of you are not putting your baby naked into the stroller for the whole day and "feed" it parts of the stroller, the amount of chemistry getting into the baby's skin is so low it is almost non-existent even with those strollers containing bad chemicals - and this is not one of them.
All in all, don't worry. You've got yourself a very nice stroller and your babies will be just fine with it. If there would be a problem, you'd already found it, as as an older model, it is on the market for some time already, so it passed the time challenge.