There are really not that many XBee models to choose from. There are Series 1 and Series 2/2.5 models. The Series 1 are point-to-point modules - two radios talking to each other. The Series 2/2.5 modules are for networking - they find each other, and form a mesh. For your needs, the Series 1 radios will work, and are much easier to set up.
Within each series, there are regular and pro models. The difference is the range, and corresponding battery life. Regular radios have a range of 100 meters or so. The pro models go up to 300 meters, but battery life is shortened considerably.
Then, for each configuration, there are chip antennas, wire antennas, and external antennas. Chip antennas are small, but have the lowest range. Wire antennas are about 2 cm (1 inch) long, and have reasonable range. The external antenna is several inches long, and has the best range. It would look a bit strange sewn into clothing, though.
The xbees operate in serial mode. To connect one to a computer, you need a serial interface. The number of computers sporting serial ports is diminishing, rapidly. So, a USB-to-serial converter is required, to connect the xbee (a serial device) to a USB port on the computer.
Any application that can read and/or write serial data can communicate with the xbees using the USB-to-serial converter.