First of all congratulations with your offer!
Sussex is a great place for both work and study. I am myself a student and have been at Sussex for more than 3 years. I'm currently doing an MSc.
There's loads of societies on campus, which you can join.
The night-life is great, you've got loads of clubs, bar (I don't drink - but have played loads of board games at the local pubs)
It's very lively, there are two bar's on campus both Falmer bar, East slope bar which is great for pre-drinks as it it cheap, then jumping on the bus for a night out.