Is it possible to buy a Windows 10 license without the update itself?

Question

Windows

Good day community. Need advice from knowledgeable people or a hint where to read.
I work for a company where many have Windows with a different zoo and "hacked". The authorities decided to go white and buy Windows 10 Pro.
Questions:
1. Is it necessary to update all systems to 10, or can you leave it as it is (accounting, secretaries and all that .. you yourself understand how "painful" it is for them).
2. Is it necessary to enter the keys into the system itself, or will it be enough for the tax or other inspection structures to show a folder with licenses exactly corresponding to the number of workstations.
3. If it will be necessary to enter keys and completely "whitewash" workstations - will they be picked up on older versions of Windows 7 and 8.
4. Perhaps you will advise a good company where you can buy all this (even if on the rights of advertising). Ukraine, Dnepropetrovsk, if there is a representative office - great

Answer 1

The answer was received from the representative of MS. For current purchases, WinPro 10 SNGL OLP NL Legalization GetGenuine wCOA is ideal. In quantity from 5 pieces. Thanks to everyone who helped figure it out

Answer 2

You need to buy a VolumeLicense.
This version is sold for 5 or more computers, allows you to downgrade, transfer the license to another computer every six months.
It is better to activate systems with a license key. The tax office will not check the keys, but the K department will easily.