Hi,
There are no problems with long cables with low-cost digital temperature sensors like DS18B20. CAT5 is typically used. Take a look on the ArduinoInfo WIKI: http://ArduinoInfo.Info
http://arduino-info.wikispaces.com/Brick-Temperature-DS18B20
There is also a link to connecting multiple sensors on the same wire, so you COULD easily monitor more than one part of your collector or even a separate small black-plate to "anticipate" the solar heating.
It also shows individual chips and waterproof versions.
Information on power control, relays etc. here: http://arduino-info.wikispaces.com/ArduinoPower