It can be hard to know what you need to think about when you are buying translation services. The Swedish Association of Professional Translators has got a few tips on their website: http://sfoe.se/pages.aspx?r_id=43131
Here are the headlines in English. You might want to think about:
The langauge combination
What the text will be used for
The readers the translation is intended for
The subject area and the number of words
The layout
Providing an approved and clear text
The delivery time
Providing reference material
Having a contact person for the translator
Discussing adaptations of the text
Giving the translator enough time
Reviewing the translation
Building a professional relationship
Working together long term
Making sure the translator only translates into his/her mother tongue
Contact the translator for changes to the text, if you need to use it for other purposes