Series 1 radios are point to point - two XBees talking to each other.
Series 2.5 radios are mesh network - a bunch of XBees passing data back and forth.
You want the Series 1 models.
The Pro versions consume more power, but the tradeoff is that they transmit further. Even the regular models are good for 300 feet.
2AA battery 5V to 5V Step Up
Don't. Get 4AA and run the Arduino/XBee properly. The batteries will last far longer, and there will be one less piece of hardware to deal with.
9V battery adapter
As long as that adapter lets you use 4AA batteries, it's a good choice.