IMO, both Luvs2listen and Bob_reynolds have given very good advices. For speakers, I would recommend Dynaudio sealed monitors e.g. Audience 42 sat or Contour S R, complemented with a sealed subwoofer.
Also, whether or not you will excite any room modes, it depends on how loud you are listening. For soft listening sessions you may not excite any room modes, so your square room may not be a problem after all.
Also, whether or not you will excite any room modes, it depends on how loud you are listening. For soft listening sessions you may not excite any room modes, so your square room may not be a problem after all.