I'm deciding between a MacBook Air or one of the Lenovo thinkpads. I know that you can just use the uni's computers for engineering work which I plan to do but I think a lot of finance work is used either excel. The excel on Mac's is pretty bad in comparison to the excel on Windows so I don't want to handicap myself.
Probs not.
I did ETC3400, in which the lecturer had us use eviews, which she said she only did because most students will have experience of it from other commerce subjects. So, you might be using excel, you might be using eviews, you might use something completely different. Engineering (AFAIK) usually uses matlab for things - any other programs I'm not aware of, but probably also some excel? Other potential programs you could end using are R and python.
Having said that, I would still recommend Windows. We don't know what program you'll be using, and I find that in general, the types of programs you use for this kind of work is usually either made for Windows (fun fact: I can't do my work on a Mac computer unless I boot Windows in it), or optimised for it (see: eviews), so you're probably better off that way.
BUT, if you prefer a Mac, by all means go ahead - there's no real restriction, and there are computer labs for the commerce students as well with all of their required programs already loaded on them if it turns out one of your subjects uses a program you can't get on Mac. (granted, they're a bit harder to get access to. Not as many and they fill up quick)
I don't think it will matter much.
I do economics, so I can only speak from the commerce perspective. Basically, any time you need to use a program like excel (or more complex statistical programs), you are doing the class in a computer lab anyway. So it doesn't really matter if you own a mac vs a windows computer since you are making use of Monash's hardware.
I will say that most people I see around uni seem to have Apple computers; I myself just use a shitty lenovo thinkpad that is about 8 years old and it is completely fine.
Actually, for ETC3400, we had to use a computational package for our assignments. The lecturer didn't mind which we used, but most people opted for eviews (excel wasn't strong enough).