People occasionally report isolated problems with various kindle models -- including the basic Kindle, the Voyage, the PaperWhite, and the Oasis. But, to my knowledge, there has been nothing systemic with any of those. There were some more common ailments in earlier models -- fading int he sun on the first and second generations, and an issue with battery drain on the third gen/keyboard model when used with the Amazon cover.
As to sales . . . . I don't think there's any way to predict exactly. BUT . . . .given that Valentines Day is only about a month away, if you're not in a hurry, it might be smart to wait until early February. It seems to me they have put various models on sale around then in past years.
As to what model to get . . . . They're all great at what they do, so it's really a matter of what is most important to the user.
The basic model is, obviously, the least expensive. It has a lower resolution screen than the lighted models, and no light -- but if you are reading where there is already plenty of light, that probably won't be an issue. It's pretty small and light on its own; the Amazon cover adds weight, but there are 3rd party options that are lighter. Note that you can not get it with 3G -- it's WiFi only. But you do have color options: black or white.
The PW is probably the most popular model because it's still very reasonably priced, but has a better screen and the built in light. Honestly, I'd not have one without a light at this point -- for me it doesn't even need to be on very bright to be helpful with contrast. But it's more than an ounce heavier than the basic and a bit bigger. It also has both a black and white option.
The Voyage is my favorite -- same great screen as the PW with somewhat better lighting and smaller and lighter -- almost as small and light as the basic. It also has the haptic buttons, but those are a neutral feature for me -- neither pro or con. BUT, you pay for the extra features in the smaller package: it's $80 more than the PW, $120 more than the basic.
The Oasis is obviously the top of the line model. My issue with it is purely aesthetic: I just think it's too square.
But it's very nicely engineered, and the buttons are great. Out of the cover it's extremely light, though I find it almost too small for me to hold comfortably. It's got the same screen as the PW and Voyage, but even more lights. I honestly don't see a big difference between the 6 on the Voyage and the 10 on the Oasis, however. And you pay a premium price: it's $90 more than the Voyage, but you do get a cover for that price. You have a choice of 3 colors of cover with auxiliary battery; there are third party options that do NOT have a battery, but you can't buy the Oasis without getting a cover, I don't think.
All of them hold thousands of books and all will have battery life of weeks; the Oasis claims months. I'd take that with a grain of salt. Regardless, they'll all last way WAY longer than the average phone or tablet because of the eInk screen.