In most cases, if there is something that you need to get, then it doesn't hurt to wait the next two weeks for Black Friday to see if the item you're planning to get will go on sale. But on the whole, I feel that the savings from that will not be very substantial, especially when compared to the savings from importing vs buying locally (~40%).
The savings really comes from buying and importing CPAP machines and masks (compared to buying locally). When you do so, you can add accessories to your order (tubes, filters, etc) to ship together, and you're still ahead. However, if you only need these simple accessories (ie: you're *not* getting a new machine or mask), you would generally be better off getting them from ebay instead.
A CPAP machine is not a one-off purchase. You should expect about 4-6 years of service from one, before you might need to replace it.
Do you need a backup machine? Considering it takes less than a week to get a new one if you purchase online, I don't really think so.
I too have to travel quite frequently, and have an S10 Autoset at home (had an S9 Autoset before that), and a Transcend Auto for travel/outfield use. Whether or not you need (or just want?) a second machine really depends on your specific circumstances (and how much extra cash you have in your wallet to toss around).
What I *do* highly recommend is, if you don't have a full face mask, and are currently using pillows or a nasal mask, to get one. You'll need it to continue your therapy if you're ever ill and have a congested nose.
You should also check with your insurer if you would be able to claim reimbursement if you purchase overseas and import it; although I'm not entirely sure why you would want to do so - if you're getting reimbursed, buying locally will ensure you have support and warranty service should you require it. Most of us import because it saves us money instead. I'm not so sure I would put in that much effort to wait for a sale, and jump though the various hoops to import a unit to help my insurer save money if I had one (and risk not getting reimbursed if it does not qualify...)