The trip from Milan to Florence is very rapid on one of the Freccia trains--just under 2 hours for a distance of over 200 miles. That is accomplished by making very few stops. If you opt to spend the night in a town between Milan and Florence that's not on the Freccia line, you will add very significantly to the total time you spend moving from Milan to Florence. I imagine you don't want to do that. At a quick glance it appears that all the Milan-Florence trains stop in Bologna, which has a huge historic district, unfortunately about a 20-minute walk from the trains station. Some also stop at the smaller city of Reggio d'Emilia--where I've never been, but Google Maps indicates the Reggio d'Emilia AV station is about 2 miles from the historic center. Both locations are disappointing from the standpoint of getting to your hotel quickly and then being able to walk around a bit in an interesting area.
If you're just looking for an inexpensive place to crash, Reggio d'Emilia may be best unless you'll be making this stop on a weekend. I've found weeknight hotel rates in Bologna not to be inexpensive. I do like the city very much, however.
The thing is, Reggio d'Emilia is a 44 minutes beyond Milan and just barely over an hour short of Florence. Is it really worth the hassle of stopping your journey such a short distance from where you want to be? You'll already be about 2/3 of the way to Florence (in terms of travel time).
Another possibility would be to look for a hotel in one of the smaller towns you'll be passing through on the regional train between the airport and Milano Centrale. That's almost a 1-hour ride in itself. I am totally unfamiliar with those towns. Hotels, if available, might be cheaper. Doing that would mean you'd have to start Day 2 with a more-than-2-hour ride to Florence. On the other hand, with an overnight stay north of Milan on your arrival day, you'd be pretty safe in buying the Milan-Florence ticket for Day 2 ahead of time, which would save a lot of money if you made that purchase when the Freccia tickets for your travel date initially went on sale.