Disclaimer: I am by no means an expert, I am just a student going into his 3rd year in ComSci who has been researching Masters degrees for a year or so.
So I am very interested in the Sussex course myself, and I do believe that it is the best choice for you, for a couple reasons.
Firstly, they teach you the mathematical foundation required for study in AI/Robotics/Complex Systems. I don't know how strong your mathematical ability is, but I am assuming that your Psychology course had less maths in say a Computer Science course, and most definitely less than any Engineering or Physical Science discipline. This course would definitely get you up to speed.
Secondly, there seems to be an emphasis on the biological/psychological aspects, in which I'm sure you'd thrive and find interesting. They seem to have a good research group on the topics that you're interested in too, with many sub groups within which cater to specific interests, probably something you should look into.
Thirdly, the topics covered is quite vast. You get exposed to Machine Learning, Bio/Psychological models of AI, Complex Systems, Robotics, Natural Language Processing, Artificial Life etc. So you'll get a wide range of experience.
I also do not think that you'd be disadvantaged at all for applying to PhDs with the Sussex MSc, quite the contrary I'd feel. From what (little) I understand of PhDs, your research interests and experience should be aligned with the program that you're applying to, so it seem advantageous to have a more specific Masters rather than just a general one. You would also be more well-equipped to write a good research proposal.
I do have a question for you, where did you do your bachelors and what did you get (if you don't mind me asking)? I'm just trying to evaluate that chance of me getting into the course too, haha.
Best of luck!