As far as I know you have to start with the 1st kit (newborn) and then when your child grows you buy the next size up - but the bigger sizes only contain they extension sides.
I looked at it because it's very pretty but it turns out stupidly expensive because EVERYTHING you get for it needs to be Stokke due to the strange size/shape - think blankets, mattress, etc. Again and again for each size you go up.
i also loved the look of the stokke! you can buy the mini kit or the whole thing in one go. if you get just the mini then you need to buy the extension kit which includes a new base, sides and mattress.
then theres the sheets etc which are pretty expensive.
after weighing it up we didnt buy it and bought a more expensive pram instead but they are a lovely looking cot and v sturdy.
They come up on eBay all the time. I got mine newborn size and then the extension kit for £150 mint condition. They are usually collection only, though. With regards to the mattresses/ bedding etc, again, eBay is your friend. There are loads of sellers who make new non-branded bedding in nice quality to fit the Stokke units.
And just to add to this, as other posters have advised, the newborn sleepi is used at first (in place of Moses if you want to) and then you size up with the extension kit, usually at around 6 months depending obvs on the size of your baby. Love mine. So pretty...and well-made.