You are allowed to change from Ashkenaz to Nusach Ari; this was done by hundreds of thousands.

For study, consult elder chasidim. Generally, start Tanya with volumes 3, 2, and then 1. Also, study from Derech Mitzvosecha. Approach study with Naseh Venishma: study and understanding will come in time, even if it seems difficult at first. The Yetzer Hara nowadays especially tries to stop people from learning Chassidus. https://www.torahrecordings.com/rebbe/igroskodesh/011/009/3569