What would be a good sefer that would introduce me to the foundations of chassidic thought and prepare me to learn the seforim of the early chassidic masters?
A great sefer for a beginner (and even for more advanced levels) which elaborates on many concepts in Chassidus, in a clear and organized fashion (written in Modern Hebrew) is Sugyos B'chassidus - סוגיות בחסידות. It is collected from the shiurim of R' Yoel Kahn, who is one of the main experts of Chabad Chassidus today.
I also recommend the companion to the book, Ha-Moadim B'Chassidus - המועדים בחסידות; which elaborates on the meaning of the holidays according to Chabad Chassidus. Here is a link to the preview, and here is a link to an online seller.
There is a book called שפתי חן - הקדמה לספרי חסידות by שמואל קרויס and חיים מענדלאוויטש that is meant to provide premises that come in useful in understanding hasidic teachings. As a commentary around the main text, it has excerpts from various famous books of early hasidic teachers, passages that discuss the premises that the book is introducing. Because it has these excerpts, it also serves as an interesting anthology of hasidic books that can be a springboard to choosing a book you want to learn in depth.
The books back to basics are specifically designed for beginers in mind. They follow the weekly Torah portions. Right now there are books for b'reishis, sh'mos, v'yikra, and bamidbar. They really help you understand the lekute sichos of the lubavitcher Rebbe. They are also designed to help one learn yiddish while learning chassidus as well.
Another set of books to take a look at are called chassidus mevueres which take Lekute Torah (written by the Alter Rebbe) and puts it in new clear typface, regular non rashi script, has the main text above, and below breaks each section down with an explanation as well as additional sources for posukim.
Tanya. Lots of books which help understand Tanya. Lessons in tanya, gps for the soul, and themes in tanya by rabbi Yekusiel green (original in Hebrew but also an English version) are three that come to mind
looking at the comments this may also help although I still think my first three suggestions are much better as far as introductions to chassidus
As for chassidus from the baal shem tov there isn't too much from him as his teachings were mainly said and some things were written down by others. One such sefer is kesser shem tov although I don't know of any book which helps explain this the weekly publication chayenu includes translations from it. The other thing which comes to mind is a book called tzavas harivash which has a new translation.
As for the maggid I had to search as I didn't know off hand but it appears he wrote two books which are titled Or Torah haSholeim and Magid D'Vorov L'Yaakov. I don't know of any translations of these works and not sure I would call them beginner books either.
I would recommend Kuntras Toras HaChassidus for your specific background ("an extensive background in Shas and poskim as well as mussar seforim"). The previous Lubavitcher Rebbe was answering someone who had your specific question (plus a few more).
It speaks to a broader perspective on Chassidus than "just" Chabad Chassidus, although that is obviously its focus, and sets the foundation of what it means to actually practice the method.
In English, a basic level book introducting chassidus with its fundamental terms and ideas is Flames of Faith: An Introduction to Chassidic Thought by R. Zev Reichman, based on the shiurim of Rav Moshe Wolfson Shlit"a.