Sorry, just saw this. There's no need to dress up at all, and I have found that most people just wear whatever...jeans, khakis, tee shirts, dress shirts, etc. You will be wearing a shop coat over it all anyway, so whatever you choose make sure it's comfortable for long hours of sitting. I personally never wear a tie at the bench - just something else to get in the way.
Even though this is far from being physically strenuous, you will be tired at the end of the day I expect. Most jobs do not require this level of focus for such a long period of time, and since you are new at this, you will require intense focus even for very simple tasks.
Cheers, Al
PS - not sure what level of magnification your loupe is, but you will need 4X or so for general assembly work, and something higher for specific tasks - I use 10X for things like oiling the level escapement for example. If you are getting a higher level of magnification, I recommend an aplanatic loupe to reduce distortion and eye fatigue.