|Your DHCP server will need to be within the LAN so that LAN clients can contact it.|
Yes, your ISP will have it's own DHCP server to dish out IP's to clients (you know, people like you) and probably also internal staff members PC's. This has nothing to do with your LAN however. Your LAN is a separate entity residing behind your firewall/router/NAT device (likely a SOHO router).
As far as the diagram goes, you really only want to detail your LAN. Your providers is not yours and you're not likely to get any details on it from them either even if you asked. As a rule of thumb, I stop at the providers "cloud".
With my network diagrams I do detail the providers equipment that resides in my LAN. Which is to say, their edge device. I also show the IP on our interface of said equipment (In our case, the one external connection is via the providers Cisco router. With our other external connection we have edge devices that are Nortel switches which connect to the providers edge devices which are Cisco switches).
I do know the IP's of our side of their devices, the other side is the "cloud" and I don't worry about it. Getting traffic from A to B beyond that point is their problem, not mine.
From what I see, your diagram looks good except for the attempt at too much detail with regard to the provider's equipment.