|1) Typically ISP's won't do DHCP for a LAN since it's easy enough to buy a SOHO router and have it do DHCP for you, or setup a DHCP server yourself within your LAN. For example, say you create an Windows based Active Directory domain. The DC will be able to do DHCP for your domain.|
2) Google is your friend. I suggest you research "workgroup" as versus "domain"
3) Without a specific reason for multiple subnets, there is no reason to do it. Again, google is your friend.
4) I've never seen a SOHO router that had 8 LAN ports. For more than 4 connections you would buy a switch with enough ports for what you need (and a few extra's to allow for growth) and you would connect it to the router and the clients in turn would connect to the switch.
5) You don't have to. Any computer within the domain (assuming you go with a domain) could be used for admin purposes. Admin level access (in a domain) depends on group membership and/or account level access (ie: domain admin as vs. user level accounts)
6) In a domain environment and logging into the domain with a "domain admin" level account (for example) would allow you what you need.
Switches/router = yes
Client PC's = no (too many)
Depends on the software.
This is just my opinion but, you don't seem to know enough about the technologies in question to do this job. I recommend you hire a qualified professional and pay them to set this up for you.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.