You really need to do some reading on single NIC on these forums and particularly the Wiki. Just do some searches for "single NIC"...
To correct the DHCP issue, you should just be able to change the config file - /etc/dhcp3 - defaults to itself because that is the normal config. If the segment is the same, and the same VLAN, then just modify the gateway address.
Yes, you can set up sub-interfaces - in fact it will do this automatically if you only have one NIC. But it will still configure itself as 2 different subnets (on the same segment of course) - you can override this if you want by setting the internal and external to be the same subnet, then all can communicate with each other.
There are reasons why you could want different VLANS on a single segment, and Linux can do dot1q signalling - not sure if you can enable this with the standard LMCE install and I wouldn't tinker if it doesn't! But I don't see anything in your description that would indicate that need tho... either way, if they are using the same subnet internally and externally it won't do anything for you (in fact it won't work) You seem to have contradictory needs, unless I have misunderstood what you meant.
If you want VLANs the switch needs to support them (or they will do nothing for you), and the internal and external networks need to be different subnets, even if they are the same physical LAN, but then you will need to use LMCE as the default gateway at least for the internal network.... that can work, but still don't know why you need them...