Hello and welcome to the forum! You'll find lots of good whistle advice here. As far as what is a good whistle to start with, you have a lot to chose from.
Hmm, a few favorites that are not going to break the bank but still give you a good sound, and playability? You will probably do well to check out generation high d whistles, but be aware that the consistency varies broadly with these whistles and they can almost always be (subjectively) improved with some simple tweaks. (search the forum for blue tack tweaks) (ramp/blade replacement or guitar pick tweaks are a bit more complicated) Try to get to a music store that sells these and will let you try a few out, you’ll see right away differences from whistle to whistle.
Another personal favorite is the Clark Sweetone whistles. They are conical, tunable, and cheap. Jerry freeman (a long time forum member) does some pretty nice tweaks with this whistle that amplifies all that is great with this instrument without introducing any problems. It will play at a nice volume against a concertina with out being lost or overbearing and is a decent whistle to learn breath pressures and fingering on. The mouthpiece was designed by Michael Copeland and the intonation of the conical shape makes for a very comfortable finger placement.
For a bit more, you could get a Susato. Very reliable whistles and suitable to play in a larger session and still be heard. If your husbands, concertina is very loud, this would be a great choice.
As far as Low Ds not being for beginners… ? Poppycock! I’ve never found any compelling reasons why a Low D should be more difficult than a High D whistle. They are two different instruments with different breath requirements, grip, and agility. Neither is a prerequisite for the other in my opinion. (tho’ some here will argue this) And actually because you are more inclined toward this sound, you will probably be more likely to spend the time learning to play it. Seek instruction, professional or otherwise. Play with others as often as you can to pick up tricks and habits and more importantly how the music goes. Learn the piper’s grip for this larger instrument and some good breath control and this won’t be any more difficult than any other wind instrument. Also Expect to pay more even on the less expensive models. Again, Susato is a good bargain here.