Simple, smart, clean clothes
Stick to conventional office clothing, if you want to convey efficiency. Do not waste money on expensive clothes to impress, but also do not look shabby or unkempt if you can avoid it.
You will (or should be!) judged on the quality of your work, and not on your clothing, unless it is dirty or outstandingly ill-fitting.
I am a professor of medicine. I am glad you are asking your question here, anonymously and you are not one of my students asking me in person: I would rebuke you for wasting time thinking about trivialities, and perhaps even for thinking I am so vain as to spend my every last penny on clothes! I can certainly afford far, far more expensive clothes, car and home than I have, but I don't want to.
When presenting at an external meeting, though, try to look smart. This does not have to be expensive. Wear the clothes your teachers wear in such occasions. If you don't know what they are going to wear (say tomorrow), then make an estimate and "round up". If you are slightly overdressed, nobody will notice. If you are markedly under-dressed, it might attract unfavourable attention.
In my field, the most cost-effective approach for students who don't want to spend much is to have a standard interview suit, and use it for presentations in front of big groups or external audiences, and for interviews. And then stop thinking about it. Work on your research instead.
By the way, my personal approach for myself? I always wear black trousers. They are interchangeable. Any time I need to give a presentation, I use any one of a handful of black suit jackets I keep in various places. It vaguely looks like a suit, in the sense that nobody notices it isn't, and nobody cares. The moment you start to get different coloured suits, you are in for a world of expensive fiddling around where you have to have the particular matching set, and damage to one part of one thing messes up a lot of money in one go. I save my brain cell
time for funner things than colour-matching, like StackExchange.