To echo others, weather will be your biggest challenge, but it also might be what makes the trip amazing.

March is an interesting month. It could still be winter with fresh snow coming down, it could be cold with piles of it everywhere, or it could be the start of mud season. It could be all 3 of those things given how much of northern new england you plan to visit. For your rental car, see if you can get a Subaru Legacy or Outback - they're a bit more available at new england rental counters. That'll give you a little bit more peace of mind if the roads aren't perfect. I still don't suggest driving in a snow storm if you can help it - part of what makes new englanders great at driving is the snow is knowing when not to do it.

As for 7-10 days, you're going to be on the road a lot. I mostly know Maine because I lived there. Much of what you want to see is going to be well off of the interstates once you go north of portland. Some suggestions:

• Skip Bangor and instead stay coastal once you get to Freeport or Brunswick. Unless there was something specific you wanted to see or do there, I don't think it's essential to your trip and it's also out of the way if you want to go to Camden and Rockport.
• The Maine coast is going to be pretty quiet in March. A lot of places will be closed, particularly those on the tourism path. Kennebunkport has a lot of places that are seasonal, so Dock Square may look a bit shut down. I still like the drive up Route 1 even if I don't stop anywhere though. But if the timing works, stopping in Biddeford for breakfast at Palace Diner is never a bad idea (but Biddeford itself isn't really a destination)
• Portland and the immediate vicinity have the most to do that time of year food-wise, and Portland is a pretty great food destination in and of itself - I'm biased, but as small US cities go, I don't think there's anywhere better
• If you like beer, you should stop at Maine Beer Co's tasting room in Freeport. I'd personally also suggest Austin Street Brewing, Allagash, and Lone Pine. Or you can use some of your time in Portland to go to the Great Lost Bear and have all of those on tap, and if you go on the weekend, their dirty wings which are great
• Camden and Rockland will also be quiet, as is Bar Harbor if you decide to keep driving. Beautiful places though, and they have a lot more charm without the summer crowds

If you need anything on places to eat and drink in southern maine let me know. I miss it every day!