Brad, no matter what key the sheet music is in or a YT video is in, it's most important that you know for sure what key your choir is singing it in. You must be in the same key and picking your whistle depends on knowing the key for sure.
Since you experiencing 'missing' lower notes on your D whistle, you have two choices: 1) play the melody in the higher D octave on that d whistle so you'll have the lower notes, or 2) use a lower key whistle and play in the key of D on it.
The trouble with solution 1) is that you'll be playing really high up and it may sound shrill and annoying to others unless you are a skilled and nuanced player.
Solution 2) may work well, if the choir is singing in key of D. Get a key of A whistle and play it in the key of D. (This is just like when people so often play in the key of G on their D whistle.) You would base your tonic key note on the D note you get with your top three fingers down. That way, by putting down more fingers you also have available the A, B, and C# notes that are lower in pitch than your 'home'/key note of D. That should cover most lower notes in the tune that you might be 'lacking' if your tonic note is with all fingers down as on a high D whistle.
Another thought is to play it on a LOW D big whistle but in the higher octave on it. But that seems more fussy than to simply play in key of D on an A whistle.
Hope I'm getting this right in my head!
Those irritated by grain of sand best avoid beach.
-Strumelia proverb c.1990