im going to second the "layers" but i want to add BREATHABLE...
cotton feathers and wool will keep you much warmer than poly blends ect...
ill give an example from when i worked at the zoo, it was mid december in CT a couple of the worst storms theyd seen in a long time and i was working outside most of the day.
1: cotton thermals and a comfortable fitting pair of cotton socks, too tight and youll be uncomfortable same for loose.
2: Cotton Pants or jeans, strangly with the thermals the jeans were plenty warm enough as long as kept DRY!
3: cotton t-shirt
4: fleece or wool sweater
5: good boots with a good thick tread
6: my timberland down filled coat.
7: gloves, i livke the fingerless gloves that are also mittens (you flip the top over)
8: hat that covers the ears or neck wrap/scarf.
keep extremities covered, fingers, nose, toes and ears are at the most risk.
keep extra clothes, particularly socks handy!
the only thing worse than being cold is being WET and cold so make sure you have lots of dry options.
id also keep a couple fleece blankets handy in the car.
with you doing such a long drive pack an "emergency kit" not only your roadside kit, but bottled water, granola bars ect
and be prepared not only to stop for traffic but to take breaks frequently.
driving in sno takes a toll on the eyes, if its bright out it can be blinding...if its snowing just strainign to see through it will do numbers...
take your time, keep your travel distance, keep your head, and if you feel yourself slipping PULL THROUGH...(ie if your on a hill and feel yourself sliping backwards dont stop, keep on the gas (dont gun it) and gently turn your steering wheel a little side to side, (like your elbow crawling up the hill)
we learnt this pretty quick with my little hyundai on my parents driveway. took me an hour but i got up!!! lol.
when picking up the rental at least ASK for all wheel or part time 4 wheel drive if possible, if not try for front wheel drive.
if its still in the same class most are happy to help if they can.
if no awd/4x pick up a couple bags of kitty litter, the clay kind...
put the bags equaly in the trunk over the wheels if possible (added weight = better traction) and if you do get stuck a sprinkling of the kitty litter will act for some extra grip...
if you have gps or can borrow one...id suggest it.
even if your good with direction...
least if you do get lost you can easily get out ot the pickle and if you get stranded with no idea where you are you can get your co-ordinates for the rescue
youll be fine.
lots of breathable layers, dont drive in all of them of course, and take your time
and GOOD LUCK