If you’re playing IrTrad, all you really need is a D whistle. Most of the music you hear is played in D or G (the second major scale on a D whistle).
That being said, if you’re playing solo you transpose to any key you like by chosing the appropriate whistle key and using the standard D fingerings.
You might want to experiment with a handful of other cheapies (I like the Walton’s C whistle and the Generation Eb and Bb models) but there’s no real need to go beyond the standard D models when you’re starting out.
On your second question, I don’t think there is a “best” whistle our there that suits everyone - until you’ve played a while and know what sound you prefer, “best” is a rather elusive concept. And most folks seem to agree that a good Generation-type whistle can be as good as anything out there, if you know how to play it.
That being said, in the mid-priced range Dixon, Susato, Serpent and (a bit higher) Syn and Hoover give you a wide range of sounds to experiment with. Or get some Freeman-tweeked cheapies - they’re about the same price point (and if you like the sound of a “good” cheapie, are a guaranteed way of getting one).
If you’re willing to go a bit higher, there’s even more choice, but until you’re more familiar with the range of sounds out there, it’s hard to choose - you may end up with a well made, well-in-tune, highly playable instrument that you never touch because you want something purer/chiffier/breather/more complex/fill-in-the-blanks.
I’ve got a long way to go before I’ll call myself a “good” player, but the more I practice (and the more whistles I try) the more I’ve come to realize that if I do my part I sound as good on a $5 whistle as on a $200 model. Mind you, I may prefer the $200 whistle because a like the sound, or the workmanship, or the feel in my hand, or the consistency (no need to tweek!), or fill-in-the-blanks, but if I sound bad on a $5 whistle I’ll sound bad on a $200 model too.
That being said, the D whistles I play most are a red-top Generation, a battered Serpent Sweet Polly (with paint chipping off ), a Syn, and a Humphrey narrow-Bore. And the most expensive (the Humphrey) and the cheapest (the Generation) sound the most similar.