I don't know the Canada customs rules, and they may have changed since I last brought a U.S. plane into Canada (so check for yourself). But, I have brought back three airplanes for import to Canada over the years ('flown two, containered the third), and, as a Canadian, I was required to pay the duty on the airplane as it entered Canada. I know that you state that you do not intend to import, but I recall it was simply the airplane entering the country under the ownership of a Canadian which aroused the interest of customs. Do your research before you commit.
As for the Seneca, I don't know a lot about the III's, though I know that the couple of I's I was involved with posed ownership challenges from an aging/maintenance perspective. You'll see "deals" on twins compared to singles, ask yourself why - cost to maintain and operate. In Cessna terms, you can often find a nice '70's 310 for less than a '70's 210, look at the maintenance and operating cost comparison, and the clues are there.
Luxury tax? Well... is a $100,000+ dollar airplane a luxury? I've always given advice to buyers to determine your total budget, and then spend half that on the airplane, to leave the rest for all the other costs to operate it for the first year - it seems to work out. And, I have seen some airplanes purchased, which just seem to sit (I flew over two I know of just the other day) -owner ran out of funds, airplane sits, and becomes less valuable.
It's been exactly 15 year since I have imported a plane, so maybe the rules are different now. But that one, (182Q through Vancouver) had to have the duty paid, before customs would return the log books to me. I paid the duty and taxes, and they had no more questions of me. I flew it here for months before I deregistered it (propeller over life for Canada, but U.S. still okay). Perhaps someone else here has more recent experience...