I used to be a lab scientist and jeans, t shirt and trainers was pretty much my uniform in those days - same for many of my colleagues. I would suggest casual-smart e.g. trousers and a top and flat shoes in case you end up traipsing around the department/campus. If you dress too formally you'll probably feel uncomfortable, but it's good to show you've gone to a little effort.
Do ask a lot of questions to get a feel for what the supervisor/working environment is like. Having quit one PhD halfway through and now doing another one, I'm pretty sure the supervisor-student relationship makes or breaks the experience. There are good supervisors, there are bad ones, but even with the good ones there are some who are better suited to some kinds of students than others. Try to get a feel for how much support/freedom you would get and how well this suits you.