First of all, call or email the dealer to make sure the website is current and the trailer you're looking at is available. They will answer the phone in French--be friendly and gracious and state the reason for your call in English. They will immediately switch to English or call someone who is bilingual. If you haven't crossed the border for awhile, you need a photo driver's license and birth certificate OR valid passport in both directions. Check the US Customs website about children, but I think they only need a birth certificate.
I am not familiar with buying in Quebec nor getting the trailer back into the US and your home state--two different situations actually. We did briefly look into buying a Sunline in Saskatchewan so here's what I learned.
All US and Canadian jurisdictions should have provisions for waiving taxes for non-residents. RV dealers especially should be familiar with this and know where to get the forms if they don't already have them--they should be a download from the Internet. These are fairly straight forward forms where you declare your residence and that the vehicle is in transit only. In my case I would have needed a copy of the purchase agreement (fax was ok) and I could get plates from my DOT and pay my provincial sales taxes at the same time before leaving to pick up the trailer. You'll need to check this procedure with your DOT. Also get insurance.
Bringing a trailer back to the US, where it was built, shoouulldd
be hassle free and duty free. Be aware that Canadian Sunlines were built to CSA rather than UL standards. These are mostly the same or insignificantly different. If you don't say anything, they'll probably never ask. Canadian Sunlines have a wall switch that prevents you from using the microwave and elec. HWH at the same time. My dealer said they also have a slightly longer A-frame. Either way, CSA or UL, doesn't seem to be a big deal. I know two people who bought used motorhomes in the US and brought them into Canada with no problems. Importing vehicles is a common occurrence and the key is to speak with customs agents who have done it before. You're law abiding citizens here--not criminals and drug smugglers, but you don't want to talk to a pointy headed bureaucrat either.
These 06 Sunlines would also have been purchased by the dealer when the CDN$ was worth US$0.90 or less. That means if Sunline wanted $20,000, the dealer had to have CDN$22,000. The dollars are roughly at par now, but the dealer may still want the $22,000 he theoretically still has in the unit.
Last thing--go to a big bank, preferably CDN like the Royal Bank, and determine the best way to move your money into Canada. I've lived in the US and my brother has for more than 40 years--small US banks are pretty clueless about moving money across the border and charge exorbitant fees to do so. Large ones, like Citibank and Royal Bank, have branches on both sides of the border and should be more familiar with the process.
Don't be discouraged. If this is the coach you want, you'll likely keep it a long time and it will be worth at least some inevitable hassle and running around. Just be sure you have absolutely everything in order and in writing/hard copy re border crossing before you sign a purchase agreement.