Well it looks like I’m going to play devil’s advocate here, and say that if you want to start flute and whistle at the same time, go for it.
I did it, years ago, and it didn’t seem to have any adverse effect on my playing of either.
Yes to play flute you have to learn the embouchure. So what? It’s like anything else, it just takes practice.
While I learned tunes and ornaments etc on the whistle I was learning the embouchure on the flute. As soon as I had an acceptable tone on the flute my fingers were ready and I could play a bunch of tunes.
In some ways the flute is easier to play than the whistle for me- for one thing I always found it easier to play in tune on the flute. Don’t know why. Of course you need a flute that’s in tune, and you have to play that flute long enough to get used to how to blow it properly. One of the best bits of advice I got when I started out was to get a good flute and stick with it until I could play it well.
About low D whistles, I love them, though they never perform as well as a flute. They take more air and have a worse finger stretch but produce less volume/power. It’s just the way it is.
About various makes of whistles, on this forum you’ll get an endless variety of opinions.
Basically, most whistles break down into three categories, each of which has its supporters:
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the “cheapies” Generations and their ilk
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Overtons and their ilk, having a straight narrow windway, more backpressure, and tending to have a tone variously described as musty, breathy, complex, haunting, etc etc
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Copelands, Burkes, Susatos, and others with a curved windway, with less backpressure/more freeblowing, which tend to have a pure or clear tone
Of course there are many makes which don’t conveniently fit these categories, but the majority do.
You would do well to try a variety to get a sense of what you like. For example, getting an Overton, a Burke, and a good-playing Generation of the same key would teach you more than reading a bunch of posts here.