I tried this, failed, pissed off a border guard, and wasted a not insignificant amount of money cleaning up the mess, so I recommend you act very carefully to make sure all the relevant pieces are in place before you attempt to import a vehicle.
Would the emissions test the dealerships perform be sufficient? Does Customs/Border Patrol require anything special?
No, it is not sufficient, and do not believe anything a dealer tells you on this. They want to sell a car, not help you import it. In fact there are two standards you need to meet. The first is the EPA emissions standard you referred to. Many manufacturers will place a sticker in the door or under the hood to certify compliance. The link has info on how to find it. If this is present, then you are in good shape on this one. If not, you will need a compliance letter from the manufacturer. Generally, most recent Canadian vehicles will meet this standard, but you still have to prove it.
The second standard is the DOT vehicle safety standard. This is what will likely get you, because there are lots of little details (like tire pressure sensors on vehicles made after a particular year) that are often omitted in Canada (even when the car is partly manufactured in the US and is an otherwise identical model to one sold in the US). There might also be a sticker for this one, and if not, you will need another compliance letter from the manufacturer. These letters can take a while to get, so I advise you to do it well before you plan to take the car over the border.
By 'importing' I plan to simply drive it across the local border crossing (into Michigan, not home state). Will customs prepare the proper forms and duties I need to pay at the crossing? Or is there something I should have prepared?
CBP will prepare the forms for you, but you must prepare EPA form 3520-1 and DOT form HS-7 before you come. You will also need all the usual documentation for the car; insurance, bill of sale, registration. If the car was manufactured in Canada or the US and you intend to keep it for personal use, it will likely be duty-free. If not, CBP will compute the duty.
Do I retain the Ontario plates until I'm able to title and register the car in Virginia?
Generally yes, this will work just like moving a car from one state to another, with the added complication that you will need the import form from CBP. Check the VA DMV to know how long you have before you need to do this.
I have my VA plates off my totaled car. How does that work?
This is up to the Virginia DMV, but probably irrelevant to your new vehicle, which will need to be registered first in Ontario. In some states, you return these with proof the car has been totalled. In others you keep them.
What financial burdens should I be aware of?
IF everything goes well, the car is compliant, and you don't have to pay duties, then the only extra costs will be transportation and registering the car in Virginia. If not, the costs could be significant; see my story below.
I attempted to import a fairly recent (2009; this was in 2015) Chevrolet that was manufactured partly in the US and partly in Canada and was also sold in more or less identical configurations in the US. It was a cheap car as I was quite poor at the time, but it worked. At the border, I presented the EPA certification, but had missed that I needed safety certification as well. It was Sunday afternoon, all the manufacturer's offices were closed, the crossing was in the middle of nowhere, and I was faced with being denied entry to the US (at least, if I wanted to bring my car with me). After 3-4 hours of sweet-talking the CBP manager, she agreed to provisionally import the car, subject to my providing her with the DoT documentation ASAP.
A week later, I get notified from the manufacturer that the car is not compliant due to a missing tire pressure sensor and a missing airbag sensor that was required for vehicles post-2009. The CBP manager told me I then needed to either show her proof the vehicle was being brought into compliance or had been returned to Canada within the next 2 weeks. Calling around for some shops, I am eventually quoted a price of $5000 to bring the vehicle (which I bought for CA$3000) into compliance. So instead, I had to drive the vehicle to the nearest Canadian city and find a dealer that will buy a vehicle for cash (not trade). I got 3 nos, then someone offered me $500. Faced with closing business hours, the fact that it was obvious that I needed to sell this car, and no guarantee that I could sell it for more later, I took it, rather than paying for another night's stay in an expensive city.
Moral of the story: check with the manufacturer that the vehicle is compliant to both standards before you buy. Preferably, look for both stickers on any vehicle you might buy. Or, even better if you can swing it, just buy a car in the states. You can drive it as long as you want in Canada.
