Without seeing the actual music, it’s hard to know what whistle is needed.
I’ve done a ton of transposing over the years, in playing studio gigs, orchestra gigs, etc because the music is almost always written either in the wrong key or the wrong range or both.
What specific concert-pitch notes are required? I word it this way because often the issue of “key” is open to debate and two people might end up talking circles around one another until the specific concert-pitch notes are revealed.
(I have this issue as we speak: I’m playing Sunday on a tune I was told was in C, but upon getting the music I see that the key signature indicates C but the Fs are sharp… and so it goes.)
I’m far more comfortable, on a Major tune, using a whistle upon which the tonic note is the whistle’s 4th degree, all things being equal. But it depends on the range required. One tune in C might fit best on a G whistle, while another apparently similar tune in C might fit best on a C whistle… and many’s the time I’ve played in C on my D uilleann chanter!
About the transposing, it’s standard practice amongst traditional whistle players to regard xxx xxx as D and to read sheet music in that way. Thus, a D Irish flute or whistle is pitched the same as a C orchestral flute, xxx ooo being G. xoo ooo being B, xxx xxo being E, etc etc.
A C whistle, like a Bb orchestral flute (if one exists) plays what’s on the page a full step lower.
Once the key of whistle that’s required is established, you can consider makes. Burkes are great but not all that much louder than a good Generation. “Classical” people tend to like the sound and performance of Susatos much more than “trad” people do, and they certainly are loud. The whistle that combines strong volume and great “trad” attributes is the Goldie/Overton.