Snow Tires - First time BMW owner Question

Hello lovely people !
Thanks in advance for accepting me in this forum of elite and proud owners of BMW X3.

My situation - I currently took a lease takeover from a colleague (who actually lost his job due to COVID) with only 14 more months left on the lease return.
BMW X3.

The current tire dimensions (performance summer) are -
245/45ZR20 103W 8Jx20 ET27
275/40ZR20 106W 9.5Jx20 ET43

Every shop I'm checking, people are quoting around 2000 CAD for 18" or 2300CAD for 19" Winter package.
(Honestly, we as a couple cannot afford to spend that amount of money now for a car which will be with just for just 1 season of winter).

I've couple of questions and would really need some guidance -

1. Can I just purchase used tires of 245 45 20 or 245 50 20 and put on the existing rims through Costco or any tire shop nearby?
If so, what are the risks I need to consider?

2. For the above dimensions of RIMs, what are the other tire size options for me to search in either Facebook market place or somewhere else?

3. Is it okay to drive the BMW X3 on 20" winter tires? or should I go down to 18"?

Thank you very much in advance..

God bless you and stay safe!!!