1) I need the R300 to be an AP and the Eth port to accept DHCP from our "offline" server.
If you factory default an R300 AP, it will look for a DHCP IP address on the Eth port LAN. (what do you mean "offine" DHCP server, active on the Eth of the AP though, right?)
You will need to look for the AP MAC address on your DHCP server lease table to determine it's IP address,
in order to open a browser for access, and to configure the WLAN for your clients. (we recommend assigning
a static IP, outside the DHCP server scope, so you can always access your AP instead of using DHCP)
2) so clients access the server thru the R300
If you configure a WLAN on the R300, the default VLAN 1, means "untagged" and drops your clients on the
same LAN as your AP. Therefore, the client DHCP requests, will go to the same DHCP server that gave your
AP an address, which is what I think you are asking for?