How to create users in AD server 2012 correctly?

Question

Active Directory

More recently, I began to build AD on the network, but as my hands reached the accounting department, misunderstandings appeared on how to properly build an account policy.
The structure was as follows: the main server is a router, all workstations in the local network, and server 1c, all in the same rank.
As it was without AD:
10 accountants had their local accounts on their PC (Let's say LocalUser1), using remote desktop, they connected to a server running windows server to a newly created local account on server 1c (Let's say ServerUser1), which is not located in domain as well as the server itself. And actually in the same place and worked in 1s.
What needs to be done now:
AD is raised on the router server.
Further under it in the same rank (But in different groups in order to set different policies) The server where 1C and all other PCs are located.
The question is:
Let's say I drive accountants' computers into AD, but to which account will they connect to Server 1c if there are no local accounts (ServerUser1) anymore?
And the second question is not on the topic:
Now I am raising AD, I put a lot of effort into setting up policies.
All files of domain users are stored on a separate hard disk CD (Domain Controller), I bought a 2tb HDD specifically for this.
The hardware is quite old, although I try to keep track of it, but all the equipment is desktop, that is, it is not designed to work 24/7.
How do I backup the OS and all settings (including the information on this hard drive) if I will be buying new hardware soon?

Answer 1

AD is raised on the router server

Don't do it.
HELL is better to take out on a separate piece of iron or a virtual machine.
You can make backups using the built-in win server tools.
Users will go under their domain accounts, all rights are resolved by AD groups and policies.