i would recommend the following:
shop for a room, and ask if you get points.
shop and book now. you can cancel. Milan does run out of rooms when there is a big event.
Milan runs out of 5* hotel rooms when there is an event attended by a lot of big bucks people.
the fiere will run more than one event at the same time.

when we ended up at the Pierre Milano 15 years ago, there were no rooms at any other 5* hotels that we could find.

Oct is a big conference time in italy. it is usually premium rate, and all the destination city high end hotels are booked way in advance. the conferences commit to blocks of rooms, and one is unable to book outside the conference.
you can see what is going on here:
http://www.milano24ore.net/download/...f_Fair-pdf.pdf
non of these looked really big to me(like the auto show or the gift show).

you can do a bit of quick shopping on venere.com. it is an extremely well organized site for your needs. maybe not best prices, but one can find most of the necessary information pre-sorted.