Minimal viable EEG equipment for dissertation research on BCI / BMI
I have been working with BCI since some time and would recommend you to try these ones. They all have been widely used in MS/PhD research and their results are more or less accepted everywhere:


Emotiv
NeuroSky  (Starts from 99$)
g.tec (Most accurate/Expensive one - not recommended for startups)


P.S: Last, but not least, feel free to have a look at BCI Competition datasets. I remember I used them in my semester project. Plus, if you are interested in getting the dataset of invasive BCI, I can provide you the email of respective research as well. Best of luck in your Research!!