I doubt thanks before is commonly used in this sense. Thanks in advance is frequently used. Here is some data on the matter. If we let Google expand our search for context, notice that thanks in advance has the context we want.
If it is written, then I personally don't think it is rude, since I understand that the author is trying to be polite. If it is spoken, and the speaker says it in a presumptuous manner, then it is really irritating.
However, there are people who will find it rude. They will more likely take it as a presumptuous statement.
I personally say, or write in an email, "Thank you for your consideration".
Some other alternatives include:
- Thank you for your time.
- I (would) appreciate your/any feedback.
- I appreciate your consideration.
I found a post online that addresses this matter. The author gives some other alternatives:
- Thank you for considering my request.
- I will be grateful for any help you can provide.
- I will appreciate your help with this situation.
- I hope you will be able to provide the information.