We also got a Siberian due to DS allergies. Originally we're going to get 2 cats but decided on one due to allergies - if we had two and DS reacted to them, we'd not have known if it was to both or one, and which one! He's had zero reaction.
A friend who is severely allergic to cats stopped coming to ours. Her DH loves cats and always had them until their relationship. Then she came but took medication first. Then she came and about 1.5 hours into dinner, we asked how she was doing with the medication and allergies. She suddenly realised she'd forgotten to take the medication. She'd had zero reaction. Her DH was very happy! They now also have a Siberian!
What I did to prepare was bring a kitten harness so that in the car on the way home if he wasn't happy in the carrier I could put the harness on (and lead) and have him on my knee/keep him away from the driver. This ended up happening (it was a long drive).
He then stayed in my bedroom for a few days. Put his food on one side near where he hid and litter on the other. Then let him out onto the landing, then bathroom, then downstairs. He couldn't quite manage the stairs at first.
Oh! I made the mistake of changing his food. Don't do that. Whatever the breeder feeds the cat, keep going with that until he's been there a few months. The vet said afterwards that it's really common for kittens to get sick if you change their food (I hadn't had an actual kitten since childhood). I eventually changed to low odour food for indoor cats..meaning low poo-odour! It makes a difference!
Also wait and see what he likes to sleep on. Mine likes firm surfaces, with no cushions or blankets. I think he just gets too hot,
Remember if you're taking him outside to get preventative flea treatment!