What do you suggest as a first low whistle?
A Chieftain Low E-Flat that I just happen to be selling… 
. Actually my first Low whistle was a Chieftain Gold Low G. The spacing was fine to learn with before I moved to a Low D which had larger spacing and required more breath control.
So I would suggest a Low whistle with fingering spacing that one could get use to ( Low F or Low G) and then move up to a Low D.
My first, and still only (due to funds, not necessarily preference) is a Howard. I love it! It is pretty easy to play, though I wasn’t quite as convinced when I first started. Practice helps immensely… the ‘now that I know more, my whistles seem so much better’ line. 
I highly recommend a Howard - price is good, quality is good, sound is good, finger stretch is not excessive.
A lot depends on your budget and what your intentions are. For folks on a limited budget who just want to try one out, a low cost starter whistle such as the one from Jubilee, or a used mid-range whistle are a good way to go.
For people looking to spend more money, I suggest listening to sound clips and then trying them out in person if at all possible. There are many, many makers of fine whistles, low and high. Some people prefer one sound over another.
I made my first low d and then bought a Howard low d. Now I have an Overton also. Somewhere inbetween I picked up a gereration Bb and tweeeked it. Seems like a low whistle to me.
My first low whistle, my first whistle ever actually, was an Overton Low D, nontunable. That was a couple of years ago and I kept it until recently when I “upgraded” (if you call going from Overton to Overton upgrading) to a tunable model.
Seconded, i could’nt have put it better my self, this sums me up to a tee 