Normally, to connect an RS232 serial device to TTL serial device such as an Arduino you'd need a TTL-to-RS232 converter. In this case, since the devices are send-only, it may be possible to build a DIY converter using a simple transistor.
If it was acceptable to use one XBee per sensor, your remote device could consist of the sensor, something to convert RS232 to TTL, and an XBee to transfer the serial stream wirelessly back to your central unit - I don't think you'd need an Arduino at all.
If you want to use the Arduino as a concentrator, you'd need to have several serial inputs (each converted from RS232 to TTL) which implies either using one of the bigger Arduinos with more hardware serial ports, or using the AltSoftSerial library. You'd also need to connect the XBee to the Arduino, perhaps via an XBee shield. You can see the cost and complexity mounting up, and if the reason for using concentrators is to reduce costs, it might turn out not to be any cheaper.
I don't know whether it would cope with your environment, but you might consider using something like this:http://shop.ciseco.co.uk/rf-328-bare-arduino-atmega-328-compatible-micro-board-rfu-328/
I don't see the range mentioned, but I seem to remember reading that it was in the range of tens of yards which is presumably comparable to your existing WiFi. Given the price, you might want to have a play with one of these and see whether it looks viable.