--- I'm not sure about the series 1 or 2.5, but the series 2 devices will set up a mesh. Part number XB24-Z7WIT-004 is the device I use. Runs close to US$20 most place
If you want the current generation, low-power models, the one with the wire antenna is Digi part number XB24-Z7WIT-004, and the chip antenna is XB24-Z7CIT-004.
Best prices I've found are direct from Digi,
XB24-Z7WIT-004: (picture shows the chip antenna model, just to confuse things even more)
ZigBee and XBee are the same thing.
The ZigBee radios come in several styles. The are Series 1 and Series 2 radios. There are regular and Pro versions of each series. There are 3 different antenna choices for each of the 4 styles.
The Series 1 radios are generally used for point-to-point networking. That is, each radio is configured to talk to one or more other radios (generally one), and to listen to one or more other radios (generally one). They do not form multi-path networks well.
The Series 2 radios are for mesh networking. They may be usable as point-to-point radios, but I couldn't get three to talk to each other. One could listen to a bunch of others, and the others could talk to that one, but I could not get two-way communication to work. You might have better luck.