The XBee modules will require a PC or some other device to connect to the ADSL modem, and can be USB or RS232 depending upon what ports you have avaiable on the PC or other device. The only way to do what you are asking (control motors over the internet/LAN) without using a PC is to use the XPort or something similar. Check out the XPort, SitePlayer, Microchip's Ethernet PICtail daughterboard, etc. You will most likely need to program a micro-controller or micro-controller like device. I believe the Xport is an ethernet to rs232 bridge type device, you would then connect that device to a separate micro-controller to control the motors, or for completely wireless operation the bridge device to an rs232 based XBee module.
My suggestion, since you stated earlier that you were more interested in learning how to do this than in just buying a product you can plug in and use, is by some micro-controllers (PICs since you say you have a programmer), download or buy a compiler (Basic, C, Pascal), some h-bridges, momentary switches, and other various components, and learn how to control a motor from switches connected to a micro-controller and work up from there. It will be much more rewarding to turn that motor on using something you built yourself.
If you decide to go this route just come back here with any questions and people will be glad to help.