temporary plates and insurance for some days
Yes, coz a reasonable owner won`t allow you to use his/her plates. And he/she is also going to annul the old insurance.
CZ requires also one export paper and Certificate of origin.
That`s an issue to discuss on a Czech forum. :)
what to have in my mind?
You can also be asked to pay the 2% tax.
Can someone guide me through the process
I would talk to the guy who wants to sell you that car. Ask him/her everything you have doubts about. Tell him/her you need this and that and if he/she can arrange it for you. If the owner wants to sell the car, he/she should contribute to it, too.
There is not much info about such cases in Polish or English, probably more in other languages. Do you speak Russian?