1. I believe "open title" means the previous owner did not register it. When you purchase a vehicle, normally the owner of record signs over the title that is in the seller's name to the purchaser. The way it is supposed to work, the purchaser pays sales tax plus title fee, and surrenders the old title to the State, later receiving a new title in the mail bearing the purchaser's name. This is usually all done at the DMV office or County Clerk who acts as the DMV's agent, at the same time as getting permanent license plates for yet another fee.
With "open title," it probably means money changed hands privately, and the title was signed over with the name of the purchaser left blank. I have signed titles over like this when bringing cars to the junkyard, so they can legally dispose of them any way they see fit.
When you give money to the person holding the vehicle and signed 'open' title, he gives them to you, and you register the vehicle as if it was purchased from the last registrant, cutting the state out of one or more rounds of sales tax. If you are removing a vehicle to another state, you give the title to that state instead to show where the vehicle came from, and usually pay any taxes at the destination.
2. You cannot drive anywhere without registration plates. There are temporary tags, or "paper plates" which are issued for the purpose of relocating vehicles. I think they may be 30 days, some might be as few as 10 days. These are sheets of cardboard the size of license plates that usually get taped inside the rear window. The expiration date is usually written in big letters with a marking pen. When you get where you are going, you throw them in the trash. You can probably get them in the state where you buy the vehicle, or you might be able to send your paperwork ahead and get them from your destination. Time permitting, you can also send your paperwork ahead and have your permanent plates in hand when you go to take possession, if you trust exchanging money and taking ownership of the bus sight unseen.
3. There are few, if any, places where you can drive without carrying proof of insurance with you. You can get insurance without plates, but almost everywhere you cannot get plates without insurance. For example, if people have a valuable car that comes off the road in the winter, they still want their policy in force to insure against fire or maybe tree limbs falling where it is parked. In the few states that didn't require liability to protect others, you wouldn't be able to cross state lines. (One state didn't require liability insurance to get plates 30 years ago, but it was against the law to have an uninsured accident there!) With New York insurance, once you put your money down with an agent, they issue a temporary insurance card which does not have a policy number, but indicates that the company has officially accepted liability for any damage or injury you might cause. Your contact will have to check on Maine procedures.
4. In addition to the proof of insurance you get for the US, make sure the agent also gets you a Canadian
proof of insurance card. It will be a second document on the same policy, but without it you may not be able to cross the border. (Disclaimer: It's been 11 years since I last drove into Canada, so it could be different now, but BE SURE TO ASK!