I went to Monza in 2005 for the LMES. We stayed in Milan as it is easy and fairly cheap to get a train from one of the stations in Milan to Monza. There is then a bus service from the station to the circuit - although we couldn't find a timetable and got bored waiting so took a taxi which I think was about 10 euros. We did walk to the circuit on the first day - but it took about an hour and not to be recommended when you then spend the day walking the circuit.
We did manage to find a bus on the way back on the second day - but I believe you have to purchase a ticket in advance - the bus driver let us on for free.
A little searching on the internet should give you details of train times etc.
Staying in Milan obviously offers the better choice of accommodation and restaurants.
HTHs
Debs
Thanks a lot
Cheers
Werner