As someone with over 10 years of professional experience in g11n, I must point out few things.
First and foremost i18n is not about extracting strings to make application translatable. Before you actually do that you should think what actual markets you will be targeting. Actually, even by your rough description of what your application will do, I can tell you that the major will be cultural fit. You may think that people tend to do business roughly the same way all other the world. And nothing could be farther from the truth.
Should you ever want to sell it outside US, you should think how to suite your target users' needs. And this is much more related to behaviors / workflows than to translation.
By the way, I don't care that much if something is translated, what I care most is how things like date, time and currency is presented. This is crucial; I can't comprehend dates like 4/11 quickly.
To answer your specific questions.
1) Frankly, I have no idea about percentage. Personally I tend to use English versions if that's possible as Polish translations tend to be horrible. Well, it is usually result of concatenating strings which does not allow for re-ordering the sentence, but still.
2) I already answered to some extent. I actually prefer everything to be in one language (be it my native Polish or English). Having part of UI in English and part translated is really irritating. And as I said, following local formats is much important.
3) This depends on your user base. In case of Poland, I believe financial / marketing people will use so many English terms that untranslated app would be easier to grasp. Should you follow local formats (or allow to change them in user preferences), that is.