It doesn't matter that it is capable of heating to 120 C because your control circuitry should prevent it from getting that high.

Unless you specifically want to do a project for your own education using an Arduino it is a lot easier to just buy a PID temperature controller such as:
http://www.ebay.co.uk/itm/Hot-PID-Di...item3d0002a096

That is jsut hte first hit on ebay, you need to read about the spec but I am quite sure that any of them will do what you want.