I'd call Brother and find out what responsibility the authorized dealers have to fix your machine even if you buy it online. I don't know about sewing machines (haven't bought one since 1994), but for many electronic devices, for warranty work, you simply need to find the authorized service center in your area. I suspect that the manufacturers give the dealers some incentive to fix electronics so they'll do it.
https://www.brother-usa.com/service/.
The other thing you can do is go to the web site I listed and find an authorized service center in your area. Call them and ask if they will fix your machine under warranty. Don't tell them that you haven't bought it yet, because of course they'll try and persuade you to buy from them. If they say yes, then you know they probably will.
Also, if you buy your machine using an American Express card, it extends your warranty for a year. Again, I have no experience testing sewing machines, but we have done a warranty claim on some stereo equipment with Amex and they were good about the claim. I'm pretty sure that Discovercard also provides similar warranty coverage (check to make sure) but I haven't tested it yet.